Fission-track Partial Annealing Zone Research Articles

Application of apatite fission-track analysis and zircon U-Pb geochronology to study the hydrothermal ore deposits in the Lesser Hinggan Range: Exhumation history and implications for mineral exploration

This study uses apatite fission track (AFT) and zircon U-Pb data to reveal the cooling history of bedrocks from the Dong'an, Cuihongshan and Pingdingshan deposits in the Lesser Hinggan Range, which is one of the most important metallogenic districts in NE China. AFT results and modeling of samples identified three cooling phases since late Mesozoic times. Stage one recorded rapid cooling to the low temperature part of the fission track partial annealing zone at circa 100 ± 10 °C during ca. 90–70 Ma with an average exhumation rate of ~29–58 m/Ma. Stage two was a relatively slow cooling period occurring between ~70 and 25 Ma with an average exhumation rate of ~12 m/Ma, suggesting that the Lesser Hinggan Range had been denuded to an area of low relief. Stage three was an increased cooling period took place from the Miocene to Quaternary (<25 Ma) with an average exhumation rate of ~47 m/Ma, which transported the sampled rocks to the present surface at an increased exhumation rate. Zircon U-Pb dating of magmatic rocks from the three deposits yielded Concordia ages of 188.8 ± 0.5 Ma (alkali-feldspar granite) and 115.6 ± 0.4 Ma (rhyolitic porphyry) at the Dong'an deposit, yielded Concordia ages of 195 ± 1 Ma (alkali-feldspar granite) and 200.3 ± 0.5 Ma (monzonitic granite) at the Cuihongshan deposit, and yielded Concordia ages of 259.4 ± 0.82 Ma (Biotite granit) and 225.9 ± 1.1 Ma (dioritic dyke) at the Pingdingshan deposit, which are notably older than their corresponding AFT ages. Our results, combined with other data in and/or around NE Asia (Great Hinggan, Changbai Mts, Yanji, Sikhote Alin Mts, South Korea and SW Japan), suggest micro-continental blocks in NE Asia underwent different uplifting/denudation rates since the Late Cretaceous, which indicates that the Xing'an, Songliao and Nadanhada-Sikhote-Alin accretionary complex blocks may have good potential for mineral exploration to great depths.

New fission-track age constraints on the exhumation of the central Santander Massif: Implications for the tectonic evolution of the Northern Andes, Colombia

The Late Cretaceous to late Neogene exhumation history of the central Santander Massif in the Northern Andes of Colombia is controlled by the geodynamic interactions between the Caribbean, South American and Nazca plates, as well as the Neogene collision and accretion of the Panama arc. Slab-breakoff of the Caribbean plate, with the tip of the slab tear presently being located beneath Bucaramanga, and the east-west oriented Caldas tear are the main structures relating seismic activity and Late Miocene to Pleistocene magmatic/hydrothermal activity and associated gold mineralization in the central Santander Massif.Here we present new apatite (AFT) and zircon fission-track (ZFT) data from 18 samples collected along two profiles in the California–Vetas block (including the Rio Charta), to the south of the Rio Charta fault, and from Bucaramanga to Picacho on the western flank of the central Santander Massif. The fission-track data are used for time–temperature history modelling and for estimating long-term average exhumation rates. The California–Vetas block in the central Santander Massif to the north of the Rio Charta fault cooled rapidly at a rate of about 24°C/Myr between 10 and 5Ma. Fast cooling was not related to post-magmatic cooling or hydrothermal activity, but rather to exhumation, with rates based on apatite fission-track cooling ages on the order of 0.3–0.4km/Myr. However, long-term average exhumation rates since the Late Cretaceous, based on zircon fission-track data, were only on the order of 0.1–0.2km/Myr. Our data indicate that next to the Rio Charta fault also the Surata fault contributed to the exhumation of the California–Vetas block.The western flank of the central Santander Massif, shows a more complete thermal history along the Bucaramanga–Picacho profile, with the exposure of an exhumed zircon fission-track partial annealing zone. Thermal history modelling of zircon fission-track data of this profile shows that after burial and heating from about 150Ma on cooling at rates of 7–10°C/Myr started at about 25Ma. For the lower part of the profile, the early Miocene ZFT ages indicate exhumation at rates of 0.3–0.5km/Myr along the Bucaramanga fault, but were only about 0.1km/Myr on the high plateau of the central Santander Massif.

複数の熱年代学的手法に基づいた江若花崗岩敦賀岩体の冷却・削剥史

In this study, multi-system thermochronology, i.e., fission-track (FT), K-Ar and U-Pb methods are used to identify the cooling and denudation history of the Tsuruga body of Kojaku granite, southwest Japan. Apatite FT age of 51.8 ± 6.5 Ma, zircon FT age of 70.4 ± 2.0 Ma, biotite K-Ar ages of 66.7–62.0 Ma, and zircon U-Pb age of 68.5 ± 0.7 Ma were obtained for granitic samples, whereas plagioclase K-Ar ages of 19.1–18.8 Ma and whole-rock K-Ar age of 19.0 ± 2.9 Ma were inferred for the basaltic dyke intruding into the granite. The zircon FT lengths are not significantly shorter than their initial lengths, implying rapid cooling at the zircon FT partial annealing zone (PAZ). On the other hand, the apatite FT length distribution shows a typical pattern for granitic pluton without reheating, indicating a slow cooling history at the apatite FT PAZ. Based on the results of these thermochronometric analyses, inverse thermal calculations using the FT data, and simple thermal conduction modeling of the granitic body, the cooling and denudation histories of the Tsuruga body are reconstructed: (1) the Tsuruga body intruded at ca. 68 Ma, late Cretaceous, at a depth of several kilometers, (2) rapidly cooled to below the zircon FT PAZ by heat conduction within a few million years or less, and (3) slowly cooled due to peneplanation during the past 50–60 million years. On the other hand, the whole-rock Rb-Sr age previously reported for the Kojaku body is younger than when the cooling curve of the Tsuruga body obtained by this study intersects with the closure temperature of the whole-rock Rb-Sr system. This may imply a time lag between the formation ages of these bodies, but more thermochronometric studies are required to draw a definitive conclusion. The K-Ar ages of the basaltic dyke are interpreted as its formation age, indicating that dyke intrusion was associated with the Green Tuff movement.

Late-stage foreland growth of China’s largest orogens (Qinling, Tibet): Evidence from the Hannan-Micang crystalline massifs and the northern Sichuan Basin, central China

This paper addresses the timing of final foreland growth of China’s largest orogens: the Mesozoic Qin Mountains (Qinling) and the Cenozoic Tibetan Plateau. In particular, we ask when the front of the Qinling orogen fold-thrust belt was emplaced, and when the northern Sichuan Basin was affected by the eastward growth of the Tibetan Plateau. We employ zircon and apatite fission-track and (U-Th)/He dating in the Proterozoic crystalline rocks of the Hannan-Micang massifs and the sedimentary rocks of the northern Sichuan Basin. The Hannan-Micang rocks remained in the zircon fission-track partial annealing zone (240 ± 30 °C) throughout the Paleozoic–Middle Triassic (481–246 Ma). From the late Middle Jurassic (ca. 165 Ma) to the early Late Cretaceous (ca. 95 Ma), enhanced cooling and exhumation, with rates of 1.2–2.5 °C/m.y. and 0.04–0.10 mm/yr, respectively, record propagation of the Qinling orogen into its leading foreland; the timing of foreland growth is supported by sedimentologic evidence, i.e., regional variation in sediment thickness and depocenter migration. Negligible cooling and exhumation since the Late Cretaceous (ca. 95 Ma) likely mark the end of the foreland fold-thrust belt formation and the subsequent persistence of a low-relief landscape that occupied extensive parts of central China; cooling and exhumation rates of 0.38–0.70 °C/m.y. and <0.02 mm/yr characterize this tectonic stagnation period. Accelerated cooling (4–5 °C/m.y.) since the Late Miocene (13–8 Ma), derived from apatite fission-track temperature-time path models, signifies involvement of the Hannan-Micang massifs and the northern Sichuan Basin into the eastward-growing Tibetan Plateau. Their inclusion into the plateau growth initiated faulting and stripped off 1.4–2.0 km of rock from the Hannan-Micang massifs and northern Sichuan Basin.

Exhumation of the Dahinggan Mountains, NE China from the Late Mesozoic to the Cenozoic: New evidence from fission-track thermochronology

Zircon and apatite fission-track (FT) analyses were used to reveal the exhumation of Phanerozoic granitoid intrusions in the Dahinggan Mountains, northeastern China. Zircon FT ages ranging from 84 ± 5 to 97 ± 8 Ma, and apatite FT ages from 47 ± 3 to 67 ± 4 Ma with mean track lengths between 12.3 ± 2.3 and 13.3 ± 2.2 μm, respectively, were obtained for the Phanerozoic granitoids sampled along a south–south-west to north–north-east transect across the Dahinggan Mountains. Three cooling stages from the Late Mesozoic to the Cenozoic have been identified in the studied region based on FT results and modeling. Stage one was a rapid cooling period from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110 °C) during ∼90–57 Ma. Stage two was a relatively slow cooling period (∼110–60 °C) occurred between ∼57 Ma and 20 Ma, suggesting that the granitoids had been exhumed to depths of ∼1–2 km. The final stage was a cooling period (60–20 °C) occurred since the Miocene, which bring studied samples to the Earth’s surface at an accelerated exhumation rate. The estimated maximum exhumation is ∼5 km assuming a steady-state geothermal gradient of 35 °C/km. Our results indicate that this exhumation is possibly led by the Pacific plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling.

The thermo-tectonic history of the Song-Kul plateau, Kyrgyz Tien Shan: Constraints by apatite and titanite thermochronometry and zircon U/Pb dating

The Song-Kul Basin sits on a plateau at the Northern and Middle Kyrgyz Tien Shan junction. It is a lacustrine basin, occupied by Lake Song-Kul and predominantly developed on igneous basement. This basement was targeted for a multi-method chronological study to identify the different magmatic episodes responsible for basement formation and to constrain the timing of the development of its present-day morphology. Zircon U/Pb dating by LA-ICP-MS revealed four different magmatic episodes: a Late Cambrian (~ 500 Ma) island arc system, a Late Ordovician (~ 450 Ma) subduction related intrusion, an Early Permian (~ 290 Ma) collisional stage, and a Middle to Late Permian (~ 260 Ma) post-collisional magmatic pulse. Middle to Late Triassic (~ 200–230 Ma) titanite fission-track ages and Late Triassic – Early Jurassic (~ 180–210 Ma) apatite fission-track ages and thermal history modeling indicate the Song-Kul basement was already emplaced in the shallow crust at that time. An exhumed fossil apatite fission-track partial annealing zone is recognized in the bordering Song-Kul mountain ranges. The area experienced only minor post-Early Mesozoic denudation. The igneous basement was slowly brought to apatite (U–Th)/He retention temperatures in the Late Cretaceous–Palaeogene. Miocene to present reactivation of the Tien Shan does not manifestly affect this part of the orogen.

Late Mesozoic‐Cenozoic exhumation history of the Lesser Hinggan Mountains, NE China, revealed by fission track thermochronology

AbstractThis study uses zircon and apatite fission‐track (FT) analyses to reveal the exhumation history of the granitoid samples collected from the Lesser Hinggan Mountains, northeast China. A southeast to northwest transect across the Lesser Hinggan Mountains yielded zircon FT ages between 89.8 ± 5.7 and 100.4 ± 8.6 Ma, and apatite FT ages between 50.6 ± 13.8 and 74.3 ± 4.5 Ma with mean track lengths between 11.7 ± 2.0 and 12.8 ± 1.7 µm. FT results and modelling identify three stages in sample cooling history spanning the late Mesozoic and Cenozoic eras. Stage one records rapid cooling from the closure temperature of zircon FT to the high temperature part of the apatite FT partial annealing zone (∼210–110 °C) during ca. 95 to 65 Ma. Stage two records a period of relative slow cooling (∼110–60 °C) taking place between ca. 65 and 20 Ma, suggesting that the granitoids had been exhumed to the depth of ∼1−2 km. Final stage cooling (60–20 °C) occurred since the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. The maximum exhumation is more than 5 km under a steady‐state geothermal gradient of 35 °C/km. Integrated with the tectonic setting, this exhumation is possibly led by the Pacific Plate subduction combined with intracontinental orogeny associated with asthenospheric upwelling. Copyright © 2010 John Wiley &amp; Sons, Ltd.

Influence of etching conditions on apatite fission-track etch pit diameter

The temperature range of the apatite fission-track partial annealing zone (PAZ) varies as a function of the kinetic characteristics of the apatite crystal as well as the cooling rate. These kinetic characteristics are controlled largely by the chemical composition of the apatites; fission-track etch pit diameter (Dpar) has been shown to be a proxy for quantifying these characteristics. Since some annealing models explicitly use Dpar as a kinetic indicator, the question of whether Dpar varies according to either operator variability or etching conditions has serious implications. The influence of etching conditions includes both minor variations in temperature and etch time as well as larger variations between the three popular concentrations of nitric acid that are presently in use within the fission-track community: 5 and 5.5 M HNO3 at 21 °C for 20 s, and 1.6 M (7 vol.%) HNO3 at 21 °C for 45 or 50 s. We have conducted a systematic study of Dpar in two widely used apatite age standards: Durango and Fish Canyon Tuff (FCT). Samples were analyzed by at least 2 operators in 2 laboratories; over 15,000 etch pits were measured. Operator imprecision is small compared to other effects both within and between operators. The measured Dpar parameters depend primarily on etching conditions including concentration of etchant, and the time and temperature of etching. Dpar size increases linearly with time but non-linearly with temperature. Differences between Dpar size of Durango and FCT are significantly smaller for the 1.6 M etch compared to the other two etches, implying that the weak etch has less resolving power for Dpar measurements. In addition, when etching conditions are changed, confined track lengths for different apatite compositions and Dpar values co-vary in a complex fashion. Therefore, the 5 or 5.5 M etch is recommended for studies that employ Dpar as a proxy for chemistry; the 1.6 M etch and other weak etches are not recommended. We propose a linear correction for Dpar based on measuring Dpar in two widely available AFT age standards, Durango and FCT, and cross-plotting the results with those obtained by R. Donelick. The slope of the resulting curve, which passes through the origin, provides a correction factor. This system yields reasonably good corrected values for 3 test samples using the 5 M and 5.5 M HNO3 etches.

Thermal evolution of a sheared continental margin: Insights from the Ballenas transform in Baja California, Mexico

The Ballenas transform margin in central Baja California offers an unparalleled opportunity to study the thermal behaviour of a sheared continental margin during various stages of its evolution. Apatite fission track and (U–Th)/He results from two transects perpendicular to the coast reveal a pronounced latest Pliocene to Pleistocene (~ 1.8 Ma) heating event related to the Neogene opening of the Gulf of California. Proximity to a regional pre-rift unconformity indicates that samples remained at near-surface levels since Paleogene unroofing, despite having experienced reheating to maximum paleotemperatures within or above the fission track partial annealing zone. In general, maximum paleotemperatures during overprinting decrease from > 100–120 °C near the coast to below 60 °C ca. 5–8 km further inland, suggesting lateral heat flow from a source within the Gulf of California. Heat sources related to the structural development of the Ballenas transform fault, located approximately 1.5–4.5 km offshore from the two sample transects, most likely controlled the observed reheating. Overprinting patterns do not support conductive reheating due to reburial, magmatism or frictional shear. Instead, a pronounced thermal spike in between much less overprinted neighbouring samples strongly favours convective heating by hydrothermal fluids as the dominant overprinting process. Hydrothermal activity may be caused by either deep fluid circulation along newly formed shear zones of the transform fault or, more likely, magmatic leaking along the transform fault. Latest Pliocene to Pleistocene (~ 1.8 Ma) activity on the Ballenas transform fault is closely linked to extension in the Lower and Upper Delfín basins and provides a minimum age for the structural reorganisation and the relocation of extension in the northern Gulf of California. This study shows that hydrothermal activity can cause significant thermal events in a transform margin before the passage of the spreading centre. Paleotemperatures from the Ballenas transform are similar to those of other transform margins after passage of a spreading centre, which suggests that hydrothermal fluids may have an important thermal buffering effect, moderating the maximum temperatures of the ridge segments near their intersection with a continental transform margin.

The effect of long-term low-temperature exposure on apatite fission track stability: A natural annealing experiment in the deep ocean

Since studies on deep-sea cores were carried out in the early 1990s it has been known that ambient temperature may have a marked affect on apatite fission track annealing. Due to sluggish annealing kinetics, this effect cannot be quantified by laboratory annealing experiments. The unknown amount of low-temperature annealing remains one of the main uncertainties for extracting thermal histories from fission track data, particularly for samples which experienced slow cooling in shallow crustal levels. To further elucidate these uncertainties, we studied volcanogenic sediments from five deep-sea drill cores, that were exposed to maximum temperatures between ∼10° and 70 °C over geological time scales of ∼15–120 Ma. Mean track lengths (MTL) and etch pit diameters (Dpar) of all samples were measured, and the chemical composition of each grain analysed for age and track length measurements was determined by electron microprobe analysis. Thermal histories of the sampled sites were independently reconstructed, based on vitrinite reflectance measurements and/or 1D numerical modelling. These reconstructions were used to test the most widely used annealing models for their ability to predict low-temperature annealing. Our results show that long-term exposure to temperatures below the temperature range of the nominal apatite fission track partial annealing zone results in track shortening ranging between 4 and 11%. Both chlorine content and Dpar values explain the downhole annealing patterns equally well. Low chlorine apatite from one drill core revealed a systematic relation between Si-content and Dpar value. The question whether Si-substitution in apatite has direct and systematic effects on annealing properties however, cannot be addressed by our data. For samples, which remained at temperatures <30 °C, and which are low in chlorine, the Laslett et al. [Laslett G., Green P., Duddy I. and Gleadow A. (1987) Thermal annealing of fission tracks in apatite. Chem. Geol. 65, 1–13] annealing model predicts MTL up to 0.6 μm longer than those actually measured, whereas for apatites with intermediate to high chlorine content, which experienced temperatures >30 °C, the predictions of the Laslett et al. (1987) model agree with the measured MTL data within error levels. With few exceptions, predictions by the Ketcham et al. [Ketcham R., Donelick R. and Carlson W. (1999) Variability of apatite fission-track annealing kinetics. III: Extrapolation to geological time scales. Am. Mineral. 84/9, 1235–1255] annealing model are consistent with the measured data for samples which remained at temperatures below ∼30 °C. For samples which experienced maximum temperatures between ∼30 and 70 °C, and which are rich in chlorine, the Ketcham et al. (1999) model overestimates track annealing.

Mesozoic–Tertiary exhumation history of the Altai Mountains, northern Xinjiang, China: New constraints from apatite fission track data

This study uses apatite fission track (FT) analysis to constrain the exhumation history of bedrock samples collected from the Altai Mountains in northern Xinjiang, China. Samples were collected as transects across the main structures related to Palaeozoic crustal accretion events. FT results and modeling identify three stages in sample cooling history spanning the Mesozoic and Tertiary. Stage one records rapid cooling to the low temperature part of the fission track partial annealing zone circa 70 ± 10 °C. Stage two, records a period of relative stability with little if any cooling taking place between ∼75 and ∼25–20 Ma suggesting the Altai region had been reduced to an area of low relief. Support for this can be found in the adjacent Junngar Basin that received little if any sediment during this interval. Final stage cooling took place in the Miocene at an accelerated rate bringing the sampled rocks to the Earth's surface. This last stage, linked to the far field effects of the Himalayan collision, most likely generated the surface uplift and relief that define the present-day Altai Mountains.

Thermochronology of the Yidun Arc, central eastern Tibetan Plateau: constraints from 40Ar/ 39Ar K-feldspar and apatite fission track data

Four K-feldspar samples from the Yidun Arc, eastern Tibetan Plateau, were analysed by the 40Ar/ 39Ar method with the aim of recovering information on their thermal history using multiple diffusion domain (MDD) theory. Arrhenius plots for each of the samples reveal low retentivity early in the heating experiments, a property that is attributed to their recrystallised nature. This low argon retentivity appears to violate the MDD assumption that volume diffusion is the only mechanism for argon transport within the crystals, thus the thermal histories derived from these analyses are considered suspect. Nevertheless, the age spectra themselves suggest that the majority of samples had cooled below ∼200 °C prior to the Eocene collision of India with Asia. Thermal history modelling from apatite fission track analyses from the same and nearby samples shows slow cooling through the apatite fission track partial annealing zone during the Cenozoic in samples from the high elevation, low relief areas of the Yidun Arc, while samples from the major Jinsha River valley show rapid cooling through the partial annealing zone beginning in the Miocene. These results suggest that significant Cenozoic denudation has been localised and that most parts of the Yidun Arc have experienced very little denudation during the Cenozoic.

Modeling of the temperature sensitivity of the apatite (U–Th)/He thermochronometer

Apatite (U–Th)/He apparent ages will generally reflect residence for extended periods at temperatures where helium is neither quantitatively retained nor lost by diffusion. To characterize the response of apatite He ages to thermal histories involving partial He retention, we explored solutions to the He production–diffusion equation. Under thermally static conditions, the analytical solution to this equation, coupled with published diffusivity data, demonstrates that the zone of partial He retention extends from about ∼40°C to ∼85°C. This zone lies at temperatures ∼35°C cooler than the analogous fission track partial annealing zone. He ages within the partial retention zone ultimately achieve a balance between He production and loss, yielding a steady state age. Both the ultimate age and the time it takes to achieve this age are temperature dependent. For example, an apatite held at 75°C equilibrates to an age of ∼2 Ma after ∼17 Myr, regardless of whether equilibrium is approached from a higher or a lower initial He age. For representative dynamic thermal histories, we evaluated apatite He ages using a numerical solution to the ingrowth–diffusion equation. The results illustrate the sensitivity of He ages to various geologic histories and are useful for understanding He age–elevation relationships and for testing time–temperature paths derived from apatite fission track length distributions. In addition, although He diffuses rapidly from apatite at shallow crustal temperatures, modeling of ambient temperature fluctuations indicates that He ages are nearly unaffected by surficial processes.

Natural long-term annealing of the zircon fission-track system in Vienna Basin deep borehole samples: constraints upon the partial annealing zone and closure temperature

Fission-track (FT) analysis of zircon from Cretaceous and Tertiary sandstone samples in Vienna Basin boreholes has provided constraints for the thermal stability of the zircon FT system over a geological timescale. Confined track lengths and ages were measured for samples from depths of ∼ 1.5-7.5 km, the deepest with a present environmental temperature of ∼ 200°C. Mean track lengths range from 10.3 to 10.8 μm, indistinguishable within error from the unannealed reference length of 10.7 ± 0.1 μm (± 1 standard error), thus showing no sign of systematic downhole reduction. Central zircon ages are consistently older than stratigraphic ages of parent rocks, with no single-grain ages significantly younger than their respective stratigraphic ages. Such FT age and length evidence strongly suggests that the tracks have not been significantly annealed since sediment deposition. Because the present geothermal regime as well as sample burial depths have been near-constant for at least the past 5 m.y. the lower temperature limit of the zircon FT partial annealing zone is > 200°C for a heating duration of the order of 5–10 m.y. Such evidence from long-term natural annealing is compatible with a zircon FT partial annealing zone cf ∼ 200–350°C derived by the extrapolation of laboratory annealing results based on o̧nfined track length measurements, but cannot alone discriminate between different annealing models. For the zircon FT closure temperature, a perhaps over-simplified concept, these results are consistent with the oft-quoted temperature of ∼ 250°C for cooling rates of ∼ 10–100°C/m.y.

Cooling history of the northern Ford Ranges, Marie Byrd Land, West Antarctica

Thermochronologic data from the Fosdick, Phillips and Chester mountains of Marie Byrd Land, West Antarctica, have been obtained through U‐Pb analysis of monazite, 40Ar/39Ar analysis of hornblende, muscovite, biotite and K‐feldspar, and apatite fission track methods. These data were collected to test the hypothesis that high‐grade metamorphic rocks in the Fosdick Mountains occupy the footwall of a Cordilleran‐style metamorphic core complex, exhumed during the breakup of this sector of Gondwana in early Late Cretaceous time. High‐grade metamorphism of rocks exposed in the Fosdick Mountains was followed by rapid cooling starting at ∼105 Ma, during the transition from convergence to extension in the adjacent continental margin of Gondwana. Monazite, hornblende, muscovite, biotite, and K‐feldspar from the Fosdick Mountains record rapid cooling (70±30°C/m.y.) from peak metamorphic conditions of 725°–780°C at 4.3–5.6 kbar to below 165°C between 105 and 94 Ma. Subsequent slow cooling was apparently punctuated by a short period of accelerated cooling through the apatite partial annealing zone (∼110°–60°C) between ∼80 and 75 Ma. Cooling rates decreased to an average of ∼l°C/m.y. after 70 Ma. Cooling ages become progressively older to the south; metamorphic grade decreases in concert with the increasing cooling ages. The southernmost samples, from the Chester Mountains, probably cooled to below K‐feldspar closure temperature (∼165°C) before inferred reheating associated with metamorphism in the Fosdick Mountains. North of the Fosdick Mountains, Devonian Ford granodiorite in the Phillips Mountains was below K‐feldspar closure temperature by early Cretaceous time. Byrd Coast granite intrusions in the eastern Phillips Mountains and east of the Fosdick and Chester mountains were emplaced between 100 and 105 Ma, and these plutons cooled very rapidly (&gt;100°C/m.y.) to below biotite closure temperature, consistent with their epizonal character. The relationship of these granitoids to metamorphic rocks in the Fosdick Mountains is uncertain. We hypothesized the following sequence of events during the transition from convergence to extension along the Pacific margin of Gondwana. Voluminous intrusion into the lower and middle crust led to increased heat flow and high‐tem‐perature, low‐to moderate‐pressure metamorphism, forming the Fosdick metamorphic complex (FMC) exposed in the Fosdick Mountains. Decrease in strength due to intrusion and partial melting resulted in large‐scale flow, probably driven by extension‐related differential stresses. This deformation ended before the onset of rapid cooling of the FMC at ∼105 Ma. Cooling rates determined for the FMC can be modeled by decreasing the heat flux into the crust and exhuming the complex at a rate of 1.5 mm/yr. The decrease in cooling rate between closure of K‐feldspar 40Ar/39Ar at ∼94 Ma and cooling into the apatite fission track partial annealing zone by ∼80 Ma is interpreted to indicate that exhumation was at least a two‐stage process. Our observations indicate that the Fosdick and Chester mountains are part of a coherent block that was tilted ∼20° to the south during the exhumation of the FMC, probably by movement along an east trending, north dipping, normal fault between the Fosdick and Phillips mountains. The Fosdick Mountains are not a Cordilleran‐style metamorphic core complex, but the FMC provides a record of middle‐crustal processes related to the rifting of New Zealand from Gondwana in the Late Cretaceous.

Thermal history of the Kunlun Batholith, N. Tibet, and implications for uplift of the Tibetan plateau

Abstract At an average elevation of 5 km, the Tibetan plateau is the highest in the world. The timing of uplift and the mechanism by which the crust was thickened to 70 km have long been a subject for debate. Preliminary Rb-Sr and 40 Ar/ 39 Ar biotite ages combined with zircon and apatite fission track dates from the Kunlun batholith in northern Tibet allow for comparison with the previously established uplift history of southern Tibet, and inferences to be made about uplift of the plateau as a whole. A major divide in the Kunlun Terrane is identified at the Golmud Fault, across which apatite ages decrease from 100 Ma in the north to 20 Ma in the south. This is believed to be as a result of a thrusting event at 120 Ma which uplifted the Northern block and overthrust the Southern block to the south. Subsequent to this event, surface approach rates (SAR) of less than 70 m Ma -1 from biotite closure at 120 Ma to apatite cooling at 20 Ma in the Southern block, indicate a long uninterrupted period of thermal equilibration in an essentially static continental block. Mean confined track lengths of 12.5–13.5 μm in apatites suggest that samples exposed at the surface today resided in the fission track partial annealing zone prior to the onset of uplift which must have occured subsequent to 20 Ma. This indicated that at least 3–4 km have been uplifted since this time and that the onset of this phase could have been as recent as 8 Ma.