Campbell, K. E., Heizler, M. T., Frailey, C. D., Romero-Pittman, L., and Prothero, D. R., Upper Cenozoic Chronostratigraphy of the Amazon Basin Establishes New Beginning Datum for GAFI, Science (submitted).

The lack of any numerical age dates for upper Cenozoic strata capping the stratigraphic sequence in the lowland Amazon Basin has greatly hindered resolution of the geologic history of modern Amazonia.  Herein we present results of preliminary magnetostratigraphy and 40Ar/39Ar dating of  two volcanic ashes from the Madre de Dios Formation of eastern Peru that constrain the last Cenozoic cycle of deposition within lowland Amazonia to between the early late Miocene and the middle late Pliocene.  The two ash dates, 9.01±0.28 Ma and 3.12±0.02 Ma, permit correlation of major late Cenozoic erosional and depositional episodes of lowland Amazonia with Andean tectonic episodes and global sea level changes.  Formation of the modern river system and physiography of lowland Amazonia began when drainage from western Amazonia began flowing into the Atlantic, probably sometime after ~2.5 Ma.  This event may be causally related to, or independent of, Plio/Pleistocene sea level lowstands.
Ispolatov, V. O., Heizler, M. T., Norman, D. I., Geology and tectonic setting of the Solton Sary Gold District, Kyrgyzstan (Central Asia). Nevada Geological Society symposium on Economic Geology (submitted).
The Solton Sary gold district is situated at the southern flank of the Kazakhstan-Tien Shan Paleozoic orogen, between an Ordovician syn-subduction batholith belt and a major regional strike-slip fault known as the Nikolaev Line. The stratigraphic section includes Cambrian-Ordovician volcanogenic rocks metamorphosed to greenschist facies unconformably overlain by Devonian-Carboniferous terrestrial conglomerates and sandstones. Gold occurs within zones of vein silicification spatially associated with small planar intrusions of lamprophyres and syenite porphyries. Preliminary fluid inclusion studies reveal trapping temperatures of about 230°C, boiling conditions, and calculated trapping pressures of 100-300 bars. The 40Ar/39Ar ages of biotite phenocrysts from lamprophyres vary from 369 to 443 Ma. The age range is due to post-crystallization thermal resetting, incorporation of excess 40Ar, and the possible presence of xenocrysts. Hydrothermal muscovites produced 40Ar loss spectra; however most maximum ages cluster between 390 and 395 Ma. K-feldspars generated well-defined 40Ar loss age gradients between 200 and 380 Ma. The observed age spectra can be explained by a reheating with a thermal maximum of 260-300°C at ca. 330 Ma. The 40Ar/39Ar data indicate that the auriferous veins and stockworks were formed at ca. 395-390 Ma (Early Devonian), after termination of major arc magmatism. The mineralization shortly predated or was concurrent with the beginning of deposition of Devonian-Carboniferous terrestrial clastic sediments. The hydrothermal activity, abrupt onset of conglomerate deposition, and perhaps the emplacement of the lamprophyres and syenites could be broadly coeval and related to a local extension regime generated by reactivation of regional strike-slip faults. Rapid accumulation of the syntectonic sediments caused deep burial of the tectonic block with the newly formed hydrothermal system. The mineralization of the Solton Sary is older than most large gold deposits of the region, which are Carbonifereous-Permian in age. The metallogenic significance of the pre-Carboniferous regional structures needs to be reevaluated.


Parry, W.T., Wilson, P.N., Jasumback, M., and Heizler, M.T. Clay mineralogy and 40Ar/39Ar dating of phyllic and argillic alteration at Bingham, Utah. Economic Geology, (submitted).
The porphyry type Cu-Mo-Au ore deposit at Bingham, Utah is hosted by an igneous complex consisting of early monzonite, followed by quartz monzonite porphyry, and later latite porphyry and quartz latite porphyry dikes. All of these rocks have been hydrothermally altered to some degree. U-Pb age of zircon from the monzonite establishes the age of crystallization of this rock at 38.46±0.26 Ma. The age of hydrothermal alteration is constrained by 40Ar/39Ar and K-Ar ages of biotite and illite. Three hydrothermal events are recognizeable. First, 40Ar/39Ar age of the most distal sample of biotite is 38.4±0.16 Ma, indistinguishable from the U-Pb zircon age of crystallization. This sample is propylitically altered and fluid incusion temperatures of 365 to 480oC (mean 403±35oC) are above the blocking temperature of Ar in biotite so this age represents the age of propylitic alteration. Second, 40Ar/39Ar ages of hydrothermal biotite in the monzonite average 37.45±0.11 and are independent of distance from the quartz monzonite porphyry. Illite veins in the monzonite are conchordant at 37.81±0.2 Ma. Third, biotite in the quartz monzonite porphyry has 40Ar/39Ar age of 37.25±0.22 and illite and K-feldspar from the same rock have conchordant ages. The duration of episodic hydrothermal activity is thus ~1.2 Ma. Igneous events associated in time with the later hydrothermal events have not been dated at Bingham. Retention of 39Ar by illitic hydrothermal clays is related to physical particle size rather than crystallinity index, intensity ratio, or percent smectite.

 

Ressel, M. W., Noble, D. C., Volk, J. A., Lamb, J. B., Park, D., Yesilyurt, S., Conrad, J. E., and Heizler, M. T., Precious-Metal mineralization in Eocene dikes at Griffin and Meikle: Bearing on the age and origin of Gold Deposits of the Carlin Trend, Nevada. Nevada Geological Society symposium on Economic Geology (submitted).
Field relations, isotopic dating and petrographic and chemical analysis of dacite dikes show that sedimentary rock-hosted disseminated gold mineralization at the Griffin and Meikle deposits is of Eocene age and closely linked to igneous activity. A 40Ar/39Ar age of 39.210.12 Ma on biotite phenocrysts from mildly argillized dacite confirms the Eocene emplacement age of several petrographically identical dikes of porphyritic dacite that intrude the Post fault zone from Meikle to the Betze-Post deposit. The dacite dikes are variably altered and locally mineralized in both Meikle and Griffin deposits. White mica is associated with fine-grained pyrite and arsenopyrite, with smaller amounts of marcasite and carbon that formed during the earliest and most gold-rich paragenetic stage. White mica from mineralized dacite in the Griffin deposit that in varous places contains 2.6 to 8.7 ppm Au and 3 to 55 ppm Ag gives a 40Ar/39Ar isochron age of about 39 Ma, suggesting that gold mineralization closely followed dike emplacement. Later hydrothermal activity at Griffin formed comb quartz veinlets containing Ag-Sb sulfosalts (pyrargyrite, miargyrite) and native silver and still later barite veinlets and vug fillings. Paragenetic and geochemical relations of ore-grade dacite from Griffin and Meikle indicate that gold was deposited with fine-grained, As-bearing iron sulfides similar to other deposits of the trend, and that a later stage resulted in the precipitation of mineral phases containing more Ag, Sb, Se, Mo and probably W but with lower amounts of S and negligible Fe. Dacite marginal to mineralized dike or carbonate rock has been converted to a mixture of montmorillonite, quartz and carbonate with little or no pyrite, but commonly containing biotite phenocrysts. Moderately altered dacite contains abundant mixed-layer illite-montmorillonite, pyrite and carbonate. More intensely altered dacite, which commonly retains phenocrystic textures, contains quartz, white mica, pyrite, and arsenopyritecarbon, with only small amounts of clay and carbonate. Multi-element analyses show progressive gains in Carlin-type pathfinder elements, including Au, Ag, S, As, Hg, Sb, and Tl, with greater intensity of sericitic alteration. Gold closely correlates with Hg and As. Other elements that were added during quartz-sericite-pyrite alteration include sulfur and smaller quantities of Se, Mo, W, Ni, Co and Cu. The alkali and alkaline earth elements Na, Ba, Ca, Sr, and Mg and Mn were removed from altered dacite. Potassium was leached to a lesser degree. Variation in the Fe/Ti and Si/Ti ratios of mineralized dacite indicate that Fe and Si were mobile during mineralization and that significant amounts of these elements were commonly added. Mineralized rocks of the various deposits of the Carlin trend have many common paragenetic and geochemical features, which argue against multiple, widely spaced periods of gold deposition. The temporal link between Eocene intrusive activity and mineralization at the Meikle, Griffin, Betze-Post, Genesis, Deep Star and Beast deposits strongly suggests that mid-Tertiary magmatism was integral in the formation of gold deposits of the Carlin trend. Isotopic ages of Eocene dikes and relative ages as determined by crosscutting relations require that Eocene gold-related hydrothermal activity was not limited to a single event but rather was related to several discrete pulses of Eocene magmatism occurring over a period of several million years between about 42 and 36 Ma.
 

Bradley, D. C., Parrish, R., Clendenen, B., Lux, D.R., Layer, P., Heizler, M. T., and Donley, T., New geochronological evidence for the timing of early Tertiary ridge subduction in Southern Alaska, U.S.G.S. Prof. Paper (in press).

We present a new U/Pb (monazite, zircon) and 40Ar/39Ar (biotite, amphibole) ages for 10 Tertiary plutons and dikes that intrude the Chugach-Prince William accretionary complex of southern Alaska. The Sanak pluton of Sanak Island yielded ages of 61.1±0.5 Ma (zircon) and 62.7±0.35 (biotite). The Shumagin pluton of Big Koniuji Island yielded a U/Pb zircon age of 61.1±0.3 Ma. Two biotite ages from the Kodiak batholith of Kodiak Island are nearly identical at 58.3±0.2 and 57.3±2.5 Ma. Amphibole from a dike at Malina Bay, Afognak Island, is 59.3±2.2 Ma; amphibole from a dike in Seldovia Bay, Kenai Peninsula, is 57.0±0.2 Ma. The Nuka pluton, Kenai Peninsula, yielded ages of 56.0±0.5 Ma (monazite) and 54.2±0.1 (biotite). Biotite plateau ages are reported for the Aialik (52.2±0.9 Ma), Tustumena (53.2±1.1 Ma), Chernof (54.2±1.1 Ma), and Hive Island (53.4±0.4 Ma) plutons of the Kenai Peninsula. Together, these new results confirm, but refine, the previously documented along-strike diachronous age trend of near-trench magmatism during the early Tertiary. We suggest that this event began at 61 Ma at Sanak Island, 2-4 m.y. later than previously supposed. An intermediate dike near Tutka Bay, Kenai Peninsula, yielded a hornblende age of 115±2 Ma. This represents a near-trench magmatic event that had heretofore gone unrecognized on the Kenai Peninsula; correlative Early Cretaceous near-trench plutons are known from the western Chugach Mountains near Palmer.

 
 
McLemore, V.T., Munroe, E. A., Heizler, M. T. and McKee, C., Geology and evolution of the Copper Flat Porphyry-Copper and associated mineral deposits in the Hillsboro mining district, Sierra County, New Mexico; in Geology and Ore Deposits 2000, The Great Basin and Beyond: Geological Society of Nevada, Special Publication, (in press).
The Hillsboro district, in central New Mexico, is an example of the typical geologic style of the development of Laramide porphyry-copper deposits in southwestern United States. New geochemical, geochronological, and geological data, combined with earlier studies, have provided a refinement of the evolution of mineralization in the district. Past production has been predominantly from the Laramide veins and placer gold deposits, although minor production has occurred from the porphyry-copper and carbonate-hosted Pb-Zn and Ag-Mn replacement deposits. The geology of the Hillsboro district is dominated by Cretaceous andesite flows (75.4±3.5 Ma, 40Ar/39Ar), breccias, and volcaniclastic rocks that were erupted from a volcano. The Copper Flat quartz monzonite porphyry (CFQM, 74.93±0.66 Ma, 40Ar/39Ar) intruded the vent of the volcano. The unmineralized Warm Springs quartz monzonite (74.4±2.6 Ma, 40Ar/39Ar) and a third altered, unmineralized quartz monzonite intruded along fracture zones on the flanks of the volcano. Younger latite and quartz latite dikes intruded the andesite and CFQM and radiate outwards from the CFQM. The igneous rocks are part of a differentiated comagmatic suite. Alteration of the igneous rocks consists of locally intense biotite-potassic, potassic, sericitic, phyllic, and argillic alteration. Large jasperoid bodies as well as smaller skarns and marbles have replaced limestones belonging to the El Paso Formation, Fusselman Dolomite and Lake Valley Limestone in the southern part of the district. The porphyry-copper deposit is a low-grade hypogene deposit that is concentrated within a breccia pipe in the CQFM stock and contains pyrite, chalcopyrite, chalcocite, molybdenite, azurite, malachite, and cuprite. The CFQM deposit forms the center of the Hillsboro district. Trending outward radially from the CFQM are Laramide Au-Ag-Cu veins hosted by many of the latite/quartz latite dikes. Carbonate-hosted replacement deposits (Ag, Pb, Mn, V, Mo, Zn) are found in the southern and northern parts of the district, distal from the CFQM deposit. Collectively, the evidence suggests that the mineral deposits found in the Hillsboro district were formed by large, convective hydrothermal systems related to the Copper Flat volcanic/intrusive complex. The CFQM porphyry-copper deposit exhibits very little supergene alteration and enrichment, in contrast to the extensive supergene alteration and enrichment found in the porphyry-copper deposits at Santa Rita and Tyrone, New Mexico and Morenci, Arizona. This is most likely a result of less pyrite (<2%) at Copper Flat and burial of the CFQM deposit from 75 to 24 Ma, preventing any supergene enrichment from occurring.
 

Moore, J. N., Powell, T. S., Heizler, M. T., and Norman, D. I., Mineralization and hydrothermal history of the Tiwi, Phillippines geothermal system. Economic Geology (in press).

The Tiwi geothermal field is related to young volcanic activity on the southern part of Luzon Island in the Philippines. In 1992, the drilling of Matalibong-25 provided nearly 1650 m of continuous core from the geothermal reservoir. This well encountered a maximum temperature of 275oC at 1829 m and intense hydrothermal alteration. Six stages of alteration and vein mineralization were documented in the cored portion of the well. The earliest stage, which reflects the initial development of the system, is represented by the deposition of chalcedony and clays (stage 1). The transition to a high-temperature environment is marked by the appearance of sericite (stage 2) deposited by the influx of acidic steam-heated waters. Stage 3 reflects episodic cycles of fluid upwelling and boiling followed by the incursion of cooler fluids. Veins deposited by boiling fluids are filled with quartz + adularia + epidote + pyrite and base-metal sulfides whereas heating of the recharging fluids led to the deposition of calcite +/or anhydrite. Maximum temperatures of fluid-inclusions trapped in two quartz crystals deposited by the upwelling fluids range from 325o to 332oC. Temperatures varied widely during the recharge phase of these cycles. Fluid inclusions trapped in calcite and anhydrite suggest that mineral deposition occurred at temperatures ranging from 333oC to less than 270oC. Fluid-inclusion salinities indicate that seawater dominated below a depth of 1600 m whereas fresh waters dominated at shallower depths. Gaseous species trapped in these inclusions were released by crushing or thermal decrepitation and analyzed with a quadrupole mass spectrometer. CO2/CH4 and N2/Ar ratios indicate that the gaseous species trapped in calcite and anhydrite were derived primarily from meteoric and crustal sources. In contrast, gaseous species in quartz from Matalibong-25 and advanced argillic assemblages from other wells have magmatic and crustal origins. The influx of acidic steam-heated waters after the second boiling cycle resulted in the deposition of stage 4 sericite. Subsequent mineralization consisted of wairakite + epidote, followed by calcite and then actinolite during stage 5. The presence of actinolite implies that temperatures exceeded 300oC at the end of stage 5. Thermochemical modeling indicates that the modern fluids are again in equilibrium with sericite (stage 6). 40Ar/39Ar spectrum dating of stage 3 adularia from three depths has been combined with the mineral parageneses and fluid-inclusion homogenization temperatures to constrain the thermal history of the geothermal system. Taken together, these data record the deposition of adularia at ~330oC between 314 and 279 ka, minor cooling followed by reheating to produce stage 5 actinolite at ~200 to 220 ka, incursion of steam-heated waters and cooling to 235oC by ~190 ka, a long period of quiescence to ~50 ka, and finally, development of the modern thermal regime at 10 to 50 ka in response to a recent subvolcanic intrusion.

 

Siddoway, C., Givot, R., Bodle, C., and Heizler, M.T. Dynamic vs. anorogenic setting for Mesoproterozoic plutonism in the Wet Mountains, Colorado: Does the interpretation depend on level of exposure? Rocky Mt. Geology (in press).

New field investigations in the Wet Mountains of Colorado reveal structural plutonic relationships important for determination of tectonic setting for Proterozoic magmatism. The comparison of two study areas emphasizes the influence of metamorphic grade, crustal position and structural rigidity of host rocks, when assessing the syntectonic (Nyman et al., 1994) versus anorogenic (Anderson et al., 1983) setting for 1.4 Ga plutons, based on deformational fabrics. The Proterozoic gneisses and schists of the Wet Mountains host syntectonic plutons of ~1.7 Ga, ~1.4 Ga intrusions, and 2-3 generations of sills and small discordant intrusions. Mineral textures and rock fabrics provide evidence of three significant Proterozoic deformational events and a widespread thermal recrystallization phase. Prograde metamorphism during the first deformational event (M1/D1) created a penetrative foliation (S1) and promoted local growth of large cordierite porphyroblasts. The cordierite preserves remarkable relict sedimentary layering (S0), defined by fine-scale banding and size-gradation of opaque mineral inclusions. S1 was folded and transposed (D2) to form the predominant S2 foliation in the range. In the central Wet Mountains, S2 strikes NW to WSW, with NE- to NNE-plunging mineral lineations defined by amphibole and streaky biotite in migmatitic gneisses. S2 strikes ~E-W to NW and defines a regional-scale, close fold in compositionally layered metamorphic sequences along the Arkansas River Canyon, in the north. Plutonic bodies of 1.66-1.7 Ga were not commmonly folded but exhibit strong S2 fabric, indicating they are syntectonic and D2 occurred at 1.66-1.7 Ga. Two areas were targeted for detailed mapping, in order to compare the increase in metamorphic grade and change in structural style from north to south along the range. Compositionally varied, middle amphibolite-grade gneisses of the Arkansas River Canyon give way to migmatitic biotite-hornblende-plagioclase-quartz gneisses in the central Wet Mountains. Index mineral assemblages (garnet-sillimanite-biotite, cordierite-biotite-quartz, sillimanite-Kfeldspar-quartz) crystallized in a dynamic setting and indicate high temperature/low-medium pressure conditions during M2/D2. The transition between migmatitic and nonmigmatized regions is invaded by abundant Mesoprotoerozoic (~1.4 Ga) granites. 1.4 Ga granites form a discrete pluton (West McCoy Gulch, Cullers et al., 1993) and discordant smaller intrusions in the Arkansas Canyon area, but comprise extensive sills in the central and southern Wet Mountains. At the margins of some intrusions, the granites assimilated blocks of host gneisses along their margins, a textural indication that host rocks supported moderately high temperatures prior to intrusion and were readily assimilated once entrained. Granitic sheets and sills are commonly foliated, but larger plutons exhibit foliation only on their margins, suggesting that the granitic magmatism was syntectonic. Shear zones truncate or transpose regional fabrics and folds, and in these penetrative zones fold evidence and relict S1 fabric are wiped out (D3). The Five Points Gulch shear zone contains a strong lineation defined by porphyroblastic sillimanite. Its higher metamorphic grade and strong deformational fabric contrast markedly with low amphibolite grade rocks outside the shear zone, so the zone appears to be an important structural-tectonic feature. Asymmetric fabrics allow interpretation of kinematic shear sense, although a widespread metamorphic mineral overprint has extensively annealed microtextures and dynamic fabrics, making kinematic interpretation difficult outside the shear zone. This granular, generally fine-textured mineral overprint is superimposed on the penetrative S2 foliation in the Wet Mountains. Randomly-oriented porphyroblasts grew to large sizes and sharply truncate foliation. Mineral growth occurred in response to sustained temperatures > 500°C, which acted to reset hornblende chronometers to 1342±6 Ma to 1369±4 Ma (weighted mean ages) contemporaneous with granitic stocks, sills, and plutons aged 1.36-1.47 Ga. Thermal metamorphism outlasted deformation. By 1.3 Ga, the compositional and textural heterogeneity of the Proterozoic basement domain was established.
 

Timmons, M. J., Karlstrom, K. E., Dehler, C. M., and Heizler, M. T., Proterozoic multistage (~1.1 and 0.8 Ga) extension in the Grand Canyon Supergroup and establishment of northwest and north-south tectonic grains in the southwestern United States, Geol. Soc. Am. Bull. (in press).

The Grand Canyon Supergroup records a prolonged history of intracratonic rifting and sedimentation in rift basins during two or more distinct tectonic/depositional events in the late-Meso and Neoproterozoic. Based on new 40Ar/39Ar age determinations, the Mesoproterozoic Unkar Group was deposited between ~1.2 and 1.1 Ga. Unkar Group basins were created by NE-SW extension which was coincident with regional NW-SE "Grenville" contraction. The Neoproterozoic (ca. 800-742 Ma) Chuar Group was deposited synchronous with normal faulting on the Butte fault during E-W extension that is interpreted to be an intracratonic response to breakup of Rodinia and initiation of the Cordilleran rift margin. Syn-extensional deposition of the Chuar Group is documented by sedimentary patterns and structures, including a growth syncline and intraformational faults. Laramide monoclines have N and NW orientations, and displacement across these structures tectonically inverted Unkar and Chuar-age faults. We use the distribution of monoclines in the Southwest to infer the extent of Proterozoic extensional fault systems. The 1.1 Ga NW trending structures and ca. 800 to 700 Ma N-S trending extensional structures created zones of weakness and major lineaments that were later reactivated during Ancestral Rocky Mountain formation, Laramide contraction, and Tertiary extension.
 
Wannamaker, P. E., Hulen, J. B., and Heizler, M. T., Early Miocene Lamproite from the Colorado Plateau Tectonic Province, Southeastern Utah, U.S.A. J. Volcan. Geothermal Res. (in press).
Newly discovered olivine phlogopite lamproite dikes intrude Jurassic siliciclastic strata in the Green River Desert subregion of the western Colorado Plateau tectonic province in southeastern Utah. The dikes yield an age of 22 Ma both from 40Ar/39Ar step-heating of phlogopite and from isochron modeling of laser-fused sanidine. This age is similar to those of mica-rich minettes and melanephelinites of the Wasatch Plateau about 125 km northwest and within the age range of the Navajo potassic volcanic field about 150 km to the southeast. The dikes intruded a pre-existing, northwest-oriented fracture system containing previously introduced bitumen, indicating that some regional lineaments of this trend are Early Miocene or older. The dikes are highly LREE-enriched, and display lamproite-specific REE ratios and phlogopite and sanidine compositions. Incompatible element and radiogenic isotope (Nd-Sr-Pb) ratios suggest that lithospheric source material modified by ancient subduction processes, together with younger asthenospheric source components, produced the melt. Timing of the intrusion coincides with the transition from Early-Middle Cenozoic, calc-alkaline plutonism to the dominantly mafic, Basin and Range type volcanism of the Late Cenozoic. While the lamproite occurrence indicates thermal input from the mantle, model non-uniqueness for both magma source depths and geophysical structure prevents quantitative comparison of Early Miocene with present-day lithospheric thickness.
 
1999 Heizler, Matthew T., Perry, Frank V., Crowe, Bruce M., Peters, L., and Appelt, R., The age of Lathrop Wells Volcanic Center: An 40Ar/39Ar dating investigation. Journal of Geophysical Research, v. 104, 767-804.
40Ar/39Ar age spectrum analyses of basalt whole rock and partially degassed sanidine xenocrysts for the Lathrop Wells volcanic center, Nevada yield eruption ages of 77.3±6.0 and 76.6±4.9ka (2s) for the Ql1 and Ql2 flows, respectively. Basaltic samples were analyzed in 4 to 6 replicate runs and yield normally distributed plateau ages. Miocene ash-flow-tuff xenoliths were collected from the basaltic units with the majority of samples coming from the Ql2 . The sanidines from the Ql2 xenoliths yielded apparent plateau ages (n=10) ranging from 69±17 to 309±11 ka with a strong cluster (n=6) at ca. 77 ka. Because the diffusion dimension of sanidine is approximated by the physical crystal size, cracking of the sanidine following the development of an argon concentration profile may result in complex argon age spectra. Experimental studies exploring the natural and laboratory argon systematics of degassed sanidine reveal that crystal breakage and HF acid etching lead to anomalously old apparent ages. These new ages for the Ql1 and Ql2 flows are analytically indistinguishable and agree well with published eruption ages determined by 3He and 36Cl cosmogenic surface exposure dating methods. We conclude that the Ql1 and Ql2 flows are temporally equivalent. Additionally, the agreement between our eruption ages for these units and the published 76±20 ka (Zreda et al., 1993) 36Cl exposure age for the Lathrop Wells scoria cone (Qs3) suggests that the cone may be temporally equivalent to the Ql1 and Ql2 flows. We believe the simplest and most reasonable interpretation is that the volcanic center is monogenetic.
 
1999 Karlstrom, K. E., Cather, S. M., Kelley, S., Heizler, M. T., Pazzaglia, F., and Roy, M. Sandia Mountains and Rio Grande Rift: Ancestry of structures and history of deformation. New Mexico Geol. Soc. Guidebook, 50th field conference, 155-165.

The Sandia Mountains and other rift flanks of the Rio Grande rift are, in large part, the product of Laramide contractional and Miocene extensional deformations superimposed upon an already segmented crust. This paper examines the ancestry of faults and fault systems in the Sandia Mountain region. We conclude that Rio Grande Rift extension represents tectonic inversion (extensional collapse) of Laramide Rocky Mountain structures. Laramide structures were, in turn, influenced by older NE (1.65 and 1.4 Ga), NW (1.1 Ga), and N-S (0.8 Ga) lineaments and structural grains. The north-dipping ramp structure at the north end of the Sandia Mountains is an important area for interpretations of the relative importance of Laramide versus Miocene structures. We suggest that the ramp itself may have been in place as part of a Laramide monoclinal/anticlinal uplift. Reverse faults in the east flank of the mountains suggests this feature may have been a mildly positive, northern extension of the Montosa uplift that resembled a mirror image of the Nacimiento uplift. However, uplift was insufficient to cause cooling of Proterozoic basement through 60-120º C until 30 Ma, hence we infer hundreds of meters, not kilometers, of structural relief. Normal faulting in the Placitas fault system began during Laramide time in a releasing bend step between the dextral Rincon and San Francisco faults. Neogene uplift of the Sandia footwall block due to tectonic denudation on these faults caused further north tilting of the ramp area and rotation of normal faults of the Placitas fault system to very steep dips. Our interpretation suggests a multistage uplift history for the Sandia Mountain block.
 
1999 Kay, R. F., Madden, R. H., Vucetich, M. G., Carlini, A. A., Mazzoni, M. M., Re, G. H., Heizler, M. T., and Sandeman, H. Revised age of the Casamayoran South American Land Mammal Age: Climatic and biotic implications, Proc. National Academy of Science, v. 96, no. 23, 13235-13240.
The absolute ages of Eocene mammalian faunas of South America are poorly known: there are no radiometric dates from rock units containing fossil mammals. Two time-successive faunas, the Casamayoran and Mustersan South American Land Mammal ŒAges‰ (SALMAs), conventionally are regarded as representing early (55-50 Ma) and middle (45-40 Ma) Eocene, respectively. A third, the Divisaderan found only at one locality is considered to be late Eocene (1-6). We report the first radiometric age determinations and associated magnetic polarity stratigraphy for a Casamayoran age fauna. The age data indicate that the Barrancan subage of the Casamayoran is late Eocene -- 18 to 20 million years younger than hitherto supposed. This age revision constrains the timing of an adaptive shift in mammalian herbivores towards hyposdonty. Specifically, the appearance of large numbers of hypsodont taxa in South America occurred sometime between about 36 and 32 Ma (late Eocene- early Oligocene ) at about the same time other biotic and geologic evidence has suggested the Southern high latitudes experienced climatic cooling associated with glaciation of Antarctica.
 
1999 Marcoline, J. R., Heizler, M. T., Goodwin, L. B., Ralser, S., and Clarke, J. Thermal, Structural and Petrological evidence for 1400-Ma metamorphism and deformation in central New Mexico. Rocky Mt. Geol., v. 34, 93-119.

Amphibolites from the Manzano Mountains, New Mexico, include two chemically and microstructurally distinct amphibole populations which record two distinct episodes of metamorphism and deformation. Petrographic and electron microprobe studies show that early actinolite is overgrown and crosscut by younger foliation-forming hornblende. Actinolite porphyroclasts are discordant to the hornblende foliation (Se). Contacts between relict actinolite grains and hornblende overgrowths are sharp rather than gradational, indicating that they do not record a single progressive metamorphic event. The foliation recorded by the hornblende in amphibolite is regional in extent. 40Ar/39Ar geochronologic analyses on hornblende, actinolite, muscovite and biotite constrain the timing of the observed metamorphic/deformational events. Most amphiboles yield complex 40Ar/39Ar age spectra, but two hornblende concentrates give preferred ages of 1410±12 Ma and 1399.1±5.4 Ma and one actinolite has a preferred age of 1391.6±5.0 Ma suggesting cooling below ~450°C at this time. These ages are interpreted to record the timing of near peak metamorphism. Muscovite sampled over a 1.5 km vertical section shows an age discordance with the structurally highest sample being ~50 Ma older than the structurally lowest sample. This age discordance is interpreted to suggest cooling from ~300°C at 0.5°C/Ma following the peak of 1400 Ma metamorphism. Biotites from similar structural levels yield variable preferred ages which range from 1402.6±5.1 to 1267.8±5.7 Ma and corroborate the slow cooling suggested by the muscovite results. Together, the thermochronologic, structural, and petrologic data are interpreted to support a model of regional deformation, metamorphism, and mineral growth at ca. 1450-1400 Ma. These data add to a growing body of evidence from throughout the southwestern United States that plutonism at ca. 1400 Ma was not anorogenic, but rather was contemporaneous with both metamorphism and deformation.
 
1999a McLemore, V. T., McMillian, N. J., Heizler, M. T., and McKee, C. Cambrian alkaline rocks at Lobo Hill, Torrance County, New Mexico: More evidence for a Cambrian-Ordovician aulacogen. New Mexico Geol. Soc. Guidebook, 50th field conference, 247-253.
Alkali-feldspar syenite, monzonite, quartz syenite, monzogranite, lamprophyre, and carbonatite dikes that intrude Proterozoic metamorphic rocks at Lobo Hill, a small hill in the Estancia Basin, southeast of Moriarty in Torrance County, are part of a widespread Cambrian-Ordovician alkaline magmatic event that occurred throughout New Mexico and southern Colorado. The dikes are unfoliated, unmetamorphosed, fine to medium grained, 1-2 m wide, and vary in attitude from nearly flat-lying to nearly vertical. Chemically, the dikes are metaluminous and can be differentiated into two groups based on differences in mineralogy and chemistry: high-K alkali-feldspar syenites and high-Na monzonites, quartz syenites, and monzogranites. An 40Ar/39Ar biotite-plateau age of 518±5.7 Ma records the age of monzonite emplacement due to the rapid cooling of this high-level intrusion. Unlike similar alkaline rocks elsewhere in New Mexico, the Lobo Hill alkaline rocks do not have significant economic potential, except for aggregate. The magmatic compositions at Lobo Hill are consistent with those generated in a continental rift system, although geologic data such as rift-basin sediments and geophysical signatures are absent for this time period in New Mexico. Recognition of widespread Cambrian-Ordovician magmatic activity in New Mexico, evidence of relatively rapid uplift and erosion in the Florida Mountains, and the presence of carbonatites suggest that New Mexico was not a simple passive margin during the Cambrian-Ordovician, but rather experienced sufficient extension to perturb the mantle and initiate magmatism. Thus, we propose that an aulacogen, similar to the Southern Oklahoma aulacogen existed in New Mexico during Cambrian and Early Ordovician time. Alkali-feldspar syenite, monzonite, quartz syenite, monzogranite, lamprophyre, and carbonatite dikes that intrude Proterozoic metamorphic rocks at Lobo Hill, a small hill in the Estancia Basin, southeast of Moriarty in Torrance County, are part of a widespread Cambrian-Ordovician alkaline magmatic event that occurred throughout New Mexico and southern Colorado. The dikes are unfoliated, unmetamorphosed, fine to medium grained, 1-2 m wide, and vary in attitude from nearly flat-lying to nearly vertical. Chemically, the dikes are metaluminous and can be differentiated into two groups based on differences in mineralogy and chemistry: high-K alkali-feldspar syenites and high-Na monzonites, quartz syenites, and monzogranites. An 40Ar/39Ar biotite-plateau age of 518±5.7 Ma records the age of monzonite emplacement due to the rapid cooling of this high-level intrusion. Unlike similar alkaline rocks elsewhere in New Mexico, the Lobo Hill alkaline rocks do not have significant economic potential, except for aggregate. The magmatic compositions at Lobo Hill are consistent with those generated in a continental rift system, although geologic data such as rift-basin sediments and geophysical signatures are absent for this time period in New Mexico. Recognition of widespread Cambrian-Ordovician magmatic activity in New Mexico, evidence of relatively rapid uplift and erosion in the Florida Mountains, and the presence of carbonatites suggest that New Mexico was not a simple passive margin during the Cambrian-Ordovician, but rather experienced sufficient extension to perturb the mantle and initiate magmatism. Thus, we propose that an aulacogen, similar to the Southern Oklahoma aulacogen existed in New Mexico during Cambrian and Early Ordovician time
 
1999 Mclemore, V. T., Munroe, E. A., Heizler, M. T., and McKee, C., Geochemistry of the Copper Flat porphyry and associated deposits in the Hillsburo Mining District, Sierra County, New Mexico, USA. Geochemical Exploration, v. 66.

New geochemical, geochronological, and geological data, combined with earlier studies, have provided a refinement of the evolution of mineralization in the Hillsboro district in central New Mexico. Laramide (polymetallic) vein, placer gold, carbonate-hosted Ag-Mn and Pb-Zn, and porphyry-copper deposits are found in this district. The Hillsboro district is dominated by Cretaceous andesite flows (75.4±3.5 Ma), breccias, and volcaniclastic rocks that were erupted from a volcano. The mineralized Copper Flat quartz-monzonite porphyry (CFQM, 74.93±0.66 Ma) intruded the vent of the volcano. The unmineralized Warm Springs quartz monzonite (74.4±2.6 Ma) and a third altered, unmineralized quartz monzonite intruded along fracture zones on the flanks of the volcano. Younger latite and quartz latite dikes intruded the andesite and CFQM and radiate outwards from the CFQM; the polymetallic vein deposits are associated with these dikes. The igneous rocks are part of a differentiated comagmatic suite. Alteration of the igneous rocks consists of locally intense silicification, biotite, potassic, phyllic, and argillic alteration. Large jasperoid bodies have replaced the El Paso Formation, Fusselman Dolomite, Lake Valley Limestone, and Percha Shale in the southern part of the district. Many workers in the district have recognized district zoning. The low sulfur (<7%) porphyry-copper deposit forms the center. Trending radially from the CFQM are Laramide Au-Ag-Cu veins. Carbonate-hosted replacement deposits (Ag, Pb, Mn, V, Mo, Zn) are located in the southern and northern parts of the district, distal from the center. Collectively, the evidence suggests that the deposits found in the Hillsboro district were formed by multiple convective hydrothermal systems related to the Copper Flat volcanic/intrusive complex.

 
 
1999 Nelson, S.T., and Davidson, J. P., Kowallis, B.J., and Heizler, M.T., Tertiary tectonic history of the southern Andes: the subvolcanic sequence to the Tatara-San Pedro Volcanic Complex, 36°S: Geological Society of America Bulletin, v. 111, p. 1387-1404.

The Tatara-San Pedro volcanic complex, a Quaternary to Recent composite volcano in central Chile, is underlain by late Miocene (6 Ma) plutons and Tertiary metavolcanicrocks. The plutons were intruded into greenschist-facies metavolcanic rocks at a depth of about 4-5 km. Together, these rocks reveal important information about the development of continental arcs.In some cases, apparent eruptive ages of metavolcanic rocks are younger than ages of intruding plutons, requiring (a) pervasive reheating of metavolcanic rocks or (b) that metavolcanic rocks were locally derived and intruded by their own magma chambers (plutons). We have estimated uplift or denudation rates of near 1 mm/yr since late Miocene time. Since normal faults currently dominate the brittle behavior of the upper crust, uplift is probably related to magmatic addition to the crust rather than to compression and shortening. Although plutonic and metavolcanic rocks display expected arc geochemical affinities, the isotopic similarity of metavolcanic and plutonic rocks to Quaternary volcanic rocks is strong, making it difficult to use subvolcanic rocks, as upper-crustal contaminants, to evaluate the igneous evolution of the Quaternary deposits. Most plutonic, metavolcanic, and Quaternary volcanic rocks exhibit the isotopic characteristics of juvenile crust. Some samples, however, exhibit sparse but unmistakable evidence (87Sr/86Sr and 40Ar/39Ar isotopic data) for the presence of older, probably Precambrian through Triassic radiogenic crust beneath the volcano. Crustal structure in this arc can be described by a three-layer model in which a central section of Precambrian through Triassic rock is underplated by juvenile material, is intruded by juvenile material, or has juvenile material transported through it to be emplaced at or near the surface. Continued erosion of juvenile material at the surface may be balanced to some degree by continued volcanism and epizonal plutonism.

 
 
1999 Read, A. S., Karlstrom, K. E., Grambling, J.A., Bowring, S.A., Heizler, M. T., and Daniel, C., A middle-crustal cross section from the Rincon Range, northern New Mexico: Evidence for 1.68-Ga, pluton-influenced tectonism and 1.4-Ga regional metamorphism. Rocky Mt. Geol., v. 34, 67-91.
In the Rincon Range, north of Mora, New Mexico, a relatively abrupt regional change in dominant fabric orientation occurs within Paleoproterozoic rocks which are nearly continuously exposed for approximately 70 km in adjacent Laramide uplifts of the southern Sangre de Cristo Mountains. Near the village of Guadalupita, these rocks display a smooth but abrupt south-to-north change from subhorizontal to subvertical dominant foliation (S2) over a distance of approximately 2 km. This change in dominant fabric orientation coincides with a regional change in metamorphic grade from near-granulite grade (approximately 650 degrees C, 4-6 kbar) in rocks with a subhorizontal fabric to amphibolite grade ( approximately 500 degrees C, 4-6 kbar) in rocks with a subvertical fabric. The shallowly dipping S2 fabric and highest temperature assemblages are both centered around an approximately 1682-Ma granitic orthogneiss, the Guadalupita pluton, which engulfs the overturned lower limb of an approximately 15 km-scale, north-facing F1fold. Porphyroblast-matrix microstructural studies suggest that S1and S2 formed during a progressive event that was synchronous withpluton emplacement and regional metamorphism at approximately 1682 Ma. Granite emplacement and its incorporation into the core of a fold-nappe at approximately 1.68 Ga appears to have facilitated subhorizontal S2 fabric development late during the progressive S1/S2 event and heat from the granite enhanced regional metamorphic conditions to create the approximately 150 degrees C temperature gradient. However, metamorphic monazites aligned in S2 yield U-Pb dates of approximately 1421 Ma, suggesting that monazite grew during renewed tectonism that reactivated the older subhorizontal fabric during approximately 1.42-Ga regional metamorphism. Present geometries therefore reflect a superposition of major tectonometamorphic events at 1.68 and 1.42 Ga. This study suggests that: (1) large temperature gradients around plutons can cause regionally heterogeneous middle-crustal pressure-temperature-time deformation (P-T-t-D) paths; (2) plutons may both localize and be localized by subhorizontal shear zones; and (3) middle-crustal rheologies are strongly influenced by thermal weakening near plutons.
 
1998 Heizler, Matthew T. and Harrison, T.M., The thermal history of the New York basement determined from 40Ar/39Ar K-feldspar studies. Journal of Geophysical Research, v. 103, 29,795-29,814.

Constraints on the thermochronologic evolution of the New York crystalline basement are mainly restricted to the high-temperature early cooling history following 1.1 Ga Grenville metamorphism (U-Pb methods) and the late cooling history (fission track methods). The thermal history for the ~one billion years between these intervals is herein assessed using 40Ar/39Ar hornblende, muscovite, biotite and K-feldspar thermochronology. Hornblende preferred ages suggest cooling below ~450-500 °C between ca. 900 and 950 Ma. Total gas ages of biotites range from ~746-1060 Ma, and their age spectra complexity is probably related to younger thermal events and/or excess argon. A single muscovite analysis yields a preferred age of 854 Ma. Argon isotope analyses of K-feldspar provide a quantitative evaluation of the thermal history between ~175 and 350°C. Multi-diffusion domain thermochronology on highly variable K-feldspar results yield internally consistent thermal histories and, along with geologic constraints, suggest thermal maxima of 275-350 °C at ca. 700, 470-450 and 300 Ma. Because of the variations in K-feldspar argon retentivity, different samples from similar areas provide information on specific events and temperature ranges. Combining all of the K-feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences. Reheating at ~700 Ma apparently affected the entire Adirondack region with the highest temperatures occurring in the eastern part of New York. This heating event may have produced the broadly regional distribution of K-feldspar results with older apparent ages in the west/north west and younger in the east/south east, and is believed to be associated with Late Precambrian rifting during the formation of the Iapetus ocean. Local reheating in the Ordovician is inferred to have affected the eastern Adirondack Mountains. It probably resulted from the combined effects of burial during the Early Paleozoic, emplacement of Taconic thrust sheets and migration of hot fluids along normal faults during the Taconic orogeny. High paleotemperatures that were previously described for the Devonian section of eastern New York are related to maximum burial of the basement during the Carboniferous at ca. 300 Ma. Slow-cooling from ca. 300 to 180 Ma is interpreted to be related to the removal of the Carboniferous section.
 
 
 

 

 

 

 

 

Abstracts

GeoCanada2000 - meeting June, 2000 Calgary, Alberta

The thermal history of the Trans-Hudson Orogen, Canada.

Matthew T. Heizler (New Mexico Bureau of Mines) New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, Shari Kelley, Kent Condie and Stacey Perilli (New Mexico Tech) M. E. Bickford and G. L. Wortman (Syracuse University) John Lewry deceased May 1999 (University of Regina) Ric Syme, Alan Bailes, Timothy Corkery, and H. Zwanzig (Energy and Mines, Manitoba)

The processes leading to the stabilization of crustal rocks following collisional orogenesis can be investigated with a variety of techniques and at a variety of scales. We have undertaken a thermochronological study across the Paleoproterozoic Trans-Hudson orogen (THO) in Saskatchewan and Manitoba which utilizes U-Pb, 40Ar/39Ar, and apatite fission track (AFT) techniques to constrain the temperature history from ca. 700 to 100¡C. This study attempts to constrain the processes by which the several interior orogenic terranes, the foreland Superior craton, and the Archean Hearne Province hinterland, were amalgamated to form a continuous craton. Over 100 rock samples were collected along a 700 km traverse at high angle to the boundaries which define the internal terranes of the THO. To date, 120 40Ar/39Ar age spectrum, 12 U-Pb, and 40 AFT analyses have been completed and provide a data set with enough resolution to document thermal histories within individual terranes as well as the entire THO. We have determined U-Pb ages for five monazite samples and seven titanites. The monazites were mostly separated from metasedimentary rocks and were, in all cases, well-developed euhedral crystals. We analyzed either single crystals or portions of single grains. Their ages, which range from 1800 to 1825 Ma, are interpreted as times when monazites grew in the rocks at temperatures below ca. 750-800¡C. Monazite ages of 1800±2 Ma are recorded from metapelites in the Thompson Belt, on the eastern margin of the orogen, in the McLennan Belt, a belt of meta-arkoses marginal to the La Ronge Domain, and in an augen gneiss from the Black Birch Lake area in the Hearne Province hinterland. Single monazite crystals in two samples from the Kisseynew Domain, north of the Flin Flon Domain, yield a range of ages from 1803 to 1817 Ma. The ages determined are distinct outside of analytical uncertainty and presumably record prolonged, or repeated, metamorphism during this time range. The oldest monazite ages, 1820-1825 Ma, were obtained from single grains separated from a metapelite in the Wollaston Belt on the eastern margin of the Hearne Province. Thus, monazite grew in rocks of the Kisseynew and Wollaston Domains earlier than they did in McLennan Belt and Thompson Belt rocks. This may indicate that the Kisseynew and Wollaston Belt rocks represent deeper levels of the orogen, implying that they were uplifted relative to the McLennan and Thompson Belt. The titanite samples were separated from metaplutonic rocks. Ages which we interpret as the time when primary igneous titanite was recrystallized and cooled through ca. 500-550¡C, range from 1765 to 1821 Ma. In two samples we found titanite that apparently recorded a primary crystallization age, whereas other titanite grains from the same rock recorded a cooling age. In one of these samples, a granodiorite from the Glennie Domain, some titanite grains yielded an age of ca. 1838 Ma, which is close to the crystallization age (ca. 1840 Ma) of plutons in this area, whereas other grains yielded concordant ages of 1801 to 1807 Ma. In the second sample, an Archean tonalitic gneiss from the Black Birch Lake area in the western Hearne Province, brown titanite grains yielded an age of ca. 2500 Ma whereas clear titanite grains yielded a concordant age of 1765 Ma. Within the orogen proper, the oldest titanite age is 1821 Ma from a granodiorite in the La Ronge Domain near the La Ronge-Glennie boundary. The youngest titanite ages are 1785 and 1791 Ma for granodiorites in the Flin Flon-Kisseynew transition region. We interpret the range of titanite ages as recording differential cooling, and presumably uplift, associated with the tectonic history of the orogenic terranes. The hornblende, muscovite, and biotite argon results reveal distinct cooling ages for several of the THO domains. Hornblende plateau ages are ca. 1760 across the entire THO, whereas muscovite and biotite ages group with nodes of ca. 1720 and 1760 Ma for specific locations within the THO. For instance, muscovite and biotite ages are nearly concordant and range from ~1740 to 1700 Ma for samples from the western THO Rottenstone, La Ronge, Glennie, and Hanson Lake domains. Micas from the Kisseynew, Snow Lake and northern Flin Flon domains cluster at 1760 Ma. Outliers to these overall patterns occur in the low-grade metamorphic rocks of the southern Flin Flon domain and the Thompson Belt located in far eastern THO. Southern Flin Flon micas and amphiboles have somewhat complex age spectra, but are typically greater than 1800 Ma. The Thompson Belt samples have the most scattered argon ages, ranging from 1760 to 1680 Ma and there is no systematic pattern to this age distribution. Moreover, somewhat perplexing results stem from hornblende/biotite pairs which are concordant for individual samples, but differ greatly (~80 Ma) from other hornblende/biotite pairs located nearby. In addition to all of the mica and amphibole analyses, 11 K-feldspars were analyzed for multiple diffusion domain thermochronology. The samples were chosen to test for differential thermal histories across terrane boundaries and large structures such as the Guncoat thrust and Tabbernor fault zone. Many of the K-feldspars have age gradients ranging from about 900 to 1700 Ma and model thermal histories (although not unique) suggest cooling from 300¡C to 200¡C between ~1700 to 1550 Ma followed by very slow-cooling to ~125¡C by 900 Ma. There are no striking thermal history differences across major structures or domain boundaries recorded by K-feldspar argon results. Apatite fission track analyses yield the lowest temperature record of the thermal history for this study. AFT samples were collected along two traverses, starting at the craton margin in the south and sampling at ~10 km interval northward toward the craton center. One traverse along Highway 102/Highway 2 began at La Ronge, Saskatchewan, and ended near South End, Saskatchewan. The AFT ages of 419 ± 35 Ma to 523 ± 31 Ma from this traverse do not show a systematic trend with distance from the craton edge. The mean track lengths are generally 11 to 12.5 µm and indicate the erosion of 1 to 2 km of Phanerozoic section since the Cretaceous. Part of the variation in AFT age is due to differences in apatite chemistry among the different lithologies sampled, with chlorapatites yielding older apparent AFT ages. However, some of the variation may reflect real differences in cooling history related to the variable thickness of the eroded Phanerozoic section. A second traverse from Flin Flon, Manitoba to about 50 km northeast of Sherridon, Manitoba gave similar results. This traverse crosses the boundary between the Flin Flon and Kisseynew domains; no significant difference in AFT cooling history across the boundary has been detected. A short traverse across the Tabbernor fault gave AFT ages of 427 ± 26 Ma on the west side of the zone, 406 ± 23 Ma within the fault zone, and 415 ± 23 Ma on the east side of the zone. The mean track lengths are 11.4 ± 0.4 µm on either side of the fault and 12.2 ± 0.9 µm within the fault zone. Although Ordovician sedimentary rocks just south of the traverse are offset by this fault and Cretaceous rocks further south along strike have been influenced by this structure, the AFT ages do not support significant vertical motion across this feature in the Phanerozoic. This observation is consistent with the K-feldspar results discussed above. The U-Pb monazite and titanite data nearly record the timing of peak ca. 1800 Ma metamorphism across the THO. Cooling of much of the orogen from ~700 to 550¡C occurred between 1830 to 1790 Ma and perhaps as late as 1760 in the western hinterland. Argon hornblende ages record cooling through ~500¡C by 1760 Ma across the entire orogen. These highest temperature cooling ages do not coincide with terrane or domain boundaries and suggest that the orogen proper cooled essentially as a unit following peak metamorphism to ~500¡C. For much of the eastern THO, mica ages are nearly concordant with hornblende ages and thus imply cooling rates between ca. 500 to 300¡C of ~20¡C/Ma. For much of the western THO, mica ages are typically 40 Ma younger than hornblendes and indicate slower cooling of about ~5¡C/Ma over this temperature interval. The approximate break in cooling rate between the western and eastern THO coincides with the Tabbernor fault zone and may be the boundary which facilitated differential cooling (denudation) to ~300¡C. Based on the present K-feldspar data, there does not appear to be a difference in cooling history on either side of the Tabbernor fault zone which may indicate that the fault ceased to accommodate differential movement by ca. 1720 Ma. There is still much to understand about the thermal history of the THO itself and also about whether or not detailed thermochronological studies of this nature provide model constraints for cooling histories of collisional origins in general. The THO represents a massive segment of continental lithosphere which, overall, has experienced early rapid cooling followed by 100Õs of millions of years of crustal stability. Unlike orogens such as the western USA Rocky Mountains which have experienced repeated and prolonged reactivation along Precambrian structures, the major crustal boundaries defining terranes and domains in the THO have apparently not undergone reactivation. Perhaps it is simply the armoring, and therefore distal location from active margins, which allow some old orogenic terranes to resist large-scale reactivation, or perhaps there are fundamental attributes to these cratonic terranes which support development of stable crust. Thermal history analysis will undoubtedly continue to provide a vital piece of information to address craton development questions, however it is clear that multidisciplinary approaches will be required to understand crustal evolution on the scale of the Trans-Hudson orogen.