Thermal history of Australian passive margin cover sequences accreted to Timor during Late Neogene arc–continent collision,Indonesia

Paleotemperature indicators and apatite fission track analysis of Australian continental margin cover sequences accreted to the active Banda arc–continent collision indicate little to no heating during rapid late Neogene uplift and exhumation. Thermal maturation patterns of vitrinite reflectance, conodont alteration and illite crystallinity show that peak paleotemperatures (PPT) increase with stratigraphic and structural burial. The highest PPT is found in the northern hinterland of the accretionary wedge, which was beneath progressively thicker parts of the upper plate towards the north. Major discontinuities in the pattern of PPT are associated with the position of major thrust ramps such as those forming the Ramelau/Kekneno Arch (RKA). PPT for Upper Triassic to Neogene strata south of the RKA are 60–80°C, which are similar to, and in many cases lower than, correlative and age equivalent units drilled on the NW Australian Shelf. Permian to Lower Triassic sedimentary strata thrust over younger units within and north of the RKA have PPT of 100–220°C. Thrust sheets accreted beneath the upper plate have PPT approximately 90°C higher than those frontally accreted. Metamorphism of the northernmost units of these sequences yield PPT of >300°C. Thrust stacking yields an inverted thermal profile of PPT decreasing discontinuously downward and to the south (towards the foreland). The timing of PPT is constrained by apatite fission track ages from mostly Triassic continental margin cover sequences. Ages of Upper Triassic units are primarily coeval with deposition and show little evidence of thermal annealing, whereas those of Lower Triassic units are almost completely annealed and range from 1.8±0.5–19.2±9.7 Ma. The clustering of apatite fission track ages into two distinct groups indicates that the upper boundary of the partial annealing zone has remained for some time at a Triassic stratigraphic interval in the slope and rise of the NW Australian continental margin. The position of this zone on the present shelf is higher in the stratigraphic column due to the greater thickness of post-breakup shelf facies units. Thrust stacking of rise, slope and shelf units produces an inverted vertical profile of increasing apatite fission track age with depth. Lack of any long confined track lengths in apatite from all of the units requires rapid and recent exhumation of the thrust stack, which is coincident with rapid phases of Pliocene–Pleistocene exhumation documented throughout Timor. These data preclude pre-Late Miocene tectonic burial or pre-Pliocene exhumation of the NW Australian continental margin.     

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

This 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 & Sons, Ltd.     

Dating emplacement and evolution of the orogenic magmatism in the internal Western Alps: 2. The Biella Volcanic Suite

The high-pressure metamorphic rocks of the Sesia?CLanzo zone are partly covered by a volcano-sedimentary unit, the Biella Volcanic Suite. Calc-alkaline and shoshonitic lavas extruded sub-aerially on the Oligocene surface. Uranium?CLead zircon dating yields 32.44?C32.89?Ma for the eruption of the calc-alkaline lavas and therefore fixes a minimum age for the paleosurface. The Biella Volcanic Suite consists mainly of epiclastic rocks deposited in a high-energy fluvial environment and minor lava flows. The rocks of the suite display widespread post-eruption transformations. Illite and chlorite thermometry as well as fission track dating suggest a thermal overprint related to burial of the Biella Volcanic Suite. An upper crustal rigid block tilting in the area causes this burial. Hydrothermal tourmaline and ankerite veins related to the intrusion of the nearby Valle del Cervo Pluton crosscut the already tilted Biella Volcanic Suite. The intrusion age of Valle del Cervo Pluton at 30.39?±?0.50?Ma sets therefore the lower time limit for tectonic processes responsible for the tilting and burial. After the burial, the Biella Volcanic Suite remained for around 20?million years between the zircon and the apatite partial annealing zone. The apatite fission track ages spread between 16 and 20?Ma gives the time frame for the second exhumation of these units. The Biella Volcanic Suite and the adjacent rocks of the Sesia?CLanzo zone were the second time exhumed to the surface in Messinian times, after a long residence time within the apatite partial annealing zone.     

Age and Thermal History of the East Ore Section,Bayan,Obo Deposit—A Fission Track Study

Considerable attention has been paid in recent years to the study of geothermal history by using spontaneous fission tracks of 238U recorded in minerals.Apatite and zircon were used for fission track study in this paper because apatitie has been widely used as a natural geothermometer(Wang Shicheng et al., 1994) to reveal the thermal evolution of sedimentary rocks based on its low annealing temperature of fission tracks and zircon is characterized by a closing temperature above 700℃,The samples were collected from ferruginous,siliceous slate wall rock at the upper levels of the orebody and Nb-REE-Fe ores from deep tunnels.The age and thermal evolution of the orebody were discussed in terms of fission track characters and their length variations observed in the coexisting apatite and zircon in the same specimen.     

Rapid early-middle Miocene exhumation of the Kazdağ Massif (western Anatolia)

Apatite fission-track analyses indicate that the Kazda? Massif in northwestern Anatolia was exhumed above the apatite partial annealing zone between 20 and 10 Ma (i.e. early-middle Miocene), with a cluster of ages at 17–14 Ma. The structural analysis of low-angle shear zones, high-angle normal faults and strike-slip faults, as well as stratigraphic analysis of upper-plate sedimentary successions and previous radiometric ages, point to a two-stage structural evolution of the massif. The first stage -encompassing much of the rapid thermal evolution of the massif- comprised late Oligocene-early Miocene low-angle detachment faulting and the associated development of small supradetachment grabens filled with a mixture of epiclastic, volcaniclastic and volcanic rocks (Küçükkuyu Fm.). The second stage (Plio-Quaternary) has been dominated by (i) strike-slip faulting related to the westward propagation of the North Anatolian fault system and (ii) normal faulting associated with present-day extension. This later stage affected the distribution of fission-track ages but did not have a component of vertical (normal) movement large enough to exhume a new partial annealing zone. The thermochronological data presented here support the notion that Neogene extensional tectonism in the northern Aegean region has been episodic, with accelerated pulses in the early-middle Miocene and Plio-Quaternary.     

Apatite fission-track dating and low-temperature history of the Bavarian Forest (southern Bohemian Massif)

Apatite fission-track (AFT) dating applied to uplifted Variscan basement blocks of the Bavarian Forest is employed to unravel the low-temperature history of this segment of the Bohemian Massif. Twenty samples were dated and confined track lengths of four samples were measured. Most samples define Cretaceous APT ages between 110 and 82 Ma (Albian to Campanian) and three samples give older ~148–140 Ma (Jurassic–Cretaceous boundary) ages. No discernible regional age variations exist between the areas north-east and south-west of the Pfahl shear zone, but >500 m post-Jurassic and post-Cretaceous vertical offsets along this and other faults can be inferred from elevation profile analyses. The AFT ages clearly postdate the Variscan exhumation history of the Bavarian Forest. Thermal modeling reveals that the ages are best explained by a slight reheating of the basement rocks to temperatures within the apatite partial annealing zone during the middle and late Jurassic and/or by late Cretaceous marine transgression causing burial heating, which affected marginal low-lying areas of the Bohemian Massif and the Bavarian Forest. Late Jurassic period was followed by enhanced cooling through the 120–60 °C temperature interval during the subsequent exhumation phase for which denudation rates of ~100 m myr^?1 were calculated. On a regional scale, Jurassic–Cretaceous AFT ages are ubiquitous in marginal structural blocks of the Bohemian Massif and seem to reflect the exhumation of these zones more distinctly compared to central parts.     

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.     

Thermal regime of a supraglacial lake on the debris-covered Koxkar Glacier, southwest Tianshan, China

A supraglacial lake was surveyed on the Koxkar Glacier in southwest Tianshan from July to September 2007 and July to September 2008, and the temperature variation characteristics of the lake, debris and debris-free ice were analyzed at different depths to determine the thermal regimes. In addition, the discrepancies of temperature variation characteristics were investigated for different geomorphic units of the ablation zone of the Koxkar Glacier. It was found that daily temperature variation curves for deep water are V-shaped because meltwater from the glacier surface at temperatures of around 0°C feeds the lake and mixes with the relatively high-temperature surface water during the day. As the water temperature rises to approximately 4°C, the mixed water sinks and forms a low-temperature trough in the deep water of the lake in the middle of the day. The vertical lapse rate of the lake water temperature against depth (?0.33°C/m) has a magnitude lower than that of the debris (?4.29°C/m) and that of the debris-free ice (?0.38°C/m) in the Koxkar Glacier??s ablation zone. The temperature curve for the surface water largely varied between the temperature curves for the debris at depths of 0.2 and 0.5?m. The surface thermal condition of the ablation zone is significantly affected by the daily weather, and there is a limited influence in debris at a depth of 1?m and in the lake at a depth of 5?m.     

Fission-track dating of the western border of the Bohemian massif: thermochronology and tectonic implications

 Since 1985, apatite fission-track analysis was applied to more than 70 samples from surface outcrops and shallow boreholes at the western margin of the Bohemian massif. Apatite ages were determined by the grain-population method. Additional information from the frequency distributions of fully confined spontaneous tracks was used for modelling of t–T paths in the low-temperature range (<120 °C). Seven zircon samples were dated by the external detector method. Zircon ages between 283 and 215 Ma indicate unroofing during the Permian molasse stage and the Triassic. Tectonic quiescence and slow subsidence prevailed from the Jurassic until the middle Cretaceous. In the basement area south of Weiden, a Mesozoic partial annealing zone (for apatite fission tracks) is now exposed at the surface. Farther north, the basement was affected by stronger Cretaceous and Palaeogene erosion, which yielded cooling ages between 110 and 49 Ma. This second period of post-Variscan denudation was correlated to reverse faulting along the Franconian Line. Received: 30 June 1996 / Accepted: 24 October 1996     

TECTONIC STRUCTURE OF THE WESTERN BLACK SEA REGION

The lack of preserved Phanerozoic units within the Proterozoic Mount Isa Inlier of northern Australia renders it difficult to determine its Phanerozoic tectonic history. However, thermo-chronological methods provide a means for assessing this problem. Apatite fission-track data from the central and southern parts of the Inlier reveal periods of post-early Carboniferous accelerated cooling. Apatite fission-track ages vary from 235 to 390 Ma and corresponding mean track lengths range from 11.76 to 13.55 microns. These results record a protracted cooling history below about 110 ± 10° C. The earlier period of cooling revealed by the data occurred during middle Carboniferous time. The event resulted in >2 km of exhumation across the Inlier and probably was in response to intra-continental deformation associated with the Alice Springs Orogeny and tectonics in the adjacent Tasman Orogen.

A high proportion of partly annealed fission tracks in the samples suggests that rocks now exposed across the Inlier resided at the top of the apatite partial annealing zone (approximately 60° C to 70° C) following the mid-Carboniferous cooling. Modeling of the fission-track age and length parameters suggests that approximately 30° C to 50° C of cooling occurred over the past 100 Ma. Assuming a geothermal gradient of 25° C/km, this corresponds to 1.2-2.0 km of exhumation. The post-Middle Cretaceous cooling possibly is related to extensional tectonics at the southern and eastern margins of the Australian plate during the Mesozoic and Tertiary periods and to the more recent collision at the northern margin of the plate.

The spatial variation of apatite fission-track data within the Inlier indicates that the three major structural belts-the Western fold belt, Kalkadoon-Leichhardt belt, and the Eastern fold belt-exhibit similar thermal histories on a regional scale. It also indicates that the main N-S fault zones bounding the belts have not been reactivated in a vertical sense along their entire length since ～350 Ma. However, adjacent smaller-scale fault-bounded blocks within the belts demonstrate variable cooling histories, suggesting that reactivation of favorably oriented minor faults within the Inlier, including segments of the major faults, probably occurred during this time interval. Variations in apatite fission-track data along the 1994 Australian Geological Survey Organization/Australian Geodynamics Co-operation Research Center (AGSO/AGCRC) Mount Isa seismic traverse indicate that up to 1 km of vertical displacement has occurred between two major intrabelt fault zones since middle Carboniferous time.     

The recognition of multiple magmatic events and pre-existing deformation zones in metamorphic rocks as illustrated by CL signatures and numerical modelling: examples from the Ballachulish contact aureole, Scotland

The combination of cathodoluminescence (CL) analysis, temperature and temperature–time calculations, and microstructural numerical modelling offers the possibility to derive the time-resolved evolution of a metamorphic rock. This combination of techniques is applied to a natural laboratory, namely the Ballachulish contact aureole, Scotland. Analysis of the Appin Quartzite reveals that the aureole was produced by two distinct magmatic events and infiltrated by associated fluids. Developing microstructures allow us to divide the aureole into three distinct regions. Region A (0–400?m, 663°C?<?T _max?<?714°C) exhibits a three-stage grain boundary migration (GBM) evolution associated with heating, fluid I and fluid II. GBM in region B (400–700?m, 630°C?<?T _max?<?663°C) is associated with fluid II only. Region C (>700?m of contact, T _max?<?630°C) is characterised by healed intragranular cracks. The combination of CL signature analysis and numerical modelling enables us to recognise whether grain size increase occurred mainly by surface energy-driven grain growth (GG) or strain-induced grain boundary migration (SIGBM). GG and SIGBM result in either straight bands strongly associated with present-day boundaries or highly curved irregular bands that often fill entire grains, respectively. At a temperature of ~620°C, evidence for GBM is observed in the initially dry, largely undeformed quartzite samples. At this temperature, evidence for GG is sparse, whereas at ~663°C, CL signatures typical for GG are commonplace. The grain boundary network approached energy equilibrium in samples that were at least 5?ka above 620°C.     

http://www.sciencedirect.com/science/article/pii/S1674987112001521

The Tarim Basin is a representative example of the basins developed in the northwest China that are characterized by multiple stages of heating and cooling.In order to better understand its complex thermal history,apatite fission track （AFT） thermochronology was applied to borehole samples from the Tazhong Uplift Zone （TUZ）.Twelve sedimentary samples of Silurian to Triassic depositional ages were analyzed from depths coinciding with the apatite partial annealing zone （～60-120 ℃）.The AFT ages,ranging from 132 ± 7 Ma （from a Triassic sample） to 25 ± 2 Ma （from a Carboniferous sample）,are clearly younger than their depositional ages and demonstrate a total resetting of the AFT thermometer after deposition.The AFT ages vary among different tectonic belts and decrease from the No.Ten Faulted Zone （133-105 Ma） in the northwest,the Central Horst Zone in the middle （108-37 Ma）,to the East Buried Hill Zone in the south （51 25 Ma）.Given the low magnitude of post-Triassic burial heating evidenced by low vitrinite reflectance values （Ro ＜ 0.7％）,the total resetting of the AFT system is speculated to result from the hot fluid flow along the faults.Thermal effects along the faults are well documented by younger AFT ages and unimodal single grain age distributions in the vicinity of the faults.Permian-early Triassic basaltic volcanism may be responsible for the early Triassic total annealing of those samples lacking connectivity with the fault.The above arguments are supported by thermal modeling results.     

Statistical modelling of annealing kinetics of fission tracks in zircon; Reassessment of laboratory experiments

We remodel the annealing kinetics of fission tracks (FTs) in zircon to fit data from 43 laboratory experiments in three earlier publications using spontaneous FT lengths in zircon from the Nisatai Dacite. Heating temperature and time are in the ranges of 350–912 °C and 10^− 3–10⁴ h, respectively. The data sets illustrate a transition in annealing process at a certain temperature zone. In order to express the annealing behavior involving the possible transitional process with improved goodness of fit, two types of models are constructed; the hybrid-linear model that consists of the fanning-linear model at high temperature and the parallel-linear model at low temperature, connected with a transitional temperature zone, and the parallel-curvilinear model that approximates smoothly the transition in the annealing process. The validity of the extrapolation of the model to geological time scale is confirmed by comparison with some deep borehole core data. The length estimates by the hybrid-linear model agree well with the borehole data, whilst the estimates by the parallel-curvilinear model show a significant disagreement with highly annealed borehole samples although this model provides good agreement with the experimental data. For the hybrid-linear model, estimates of the partial annealing zone corresponding to isothermal heating for 10⁶ and 10⁷ years are 281–352 °C and 262–330 °C where the top and bottom of the partial annealing zone are defined by the mean length reduction ratio of 0.4 and 0.8, respectively.     

Fission-track constraints on the thermal and tectonic evolution of the Apuseni Mountains (Romania)

New zircon and apatite fission-track (FT) data, including apatite thermal modelling, are combined with an extensive literature survey and reconnaissance-type structural fieldwork in the Eastern Apuseni Mountains. This leads to a better understanding of the complex structural and thermal history of a key area at the boundary between two megatectonic units in the Balkan peninsula, namely the Tisza and Dacia Mega-Units. Following Late Jurassic obduction of the Transylvanian ophiolites onto a part of the Dacia Mega-Unit, that is, the Biharia nappe system, both units were buried to a minimum of 8 km during late Early Cretaceous times when these units were underthrust below the Tisza Mega-Unit consisting of the present-day Codru and Bihor nappe systems. Tisza formed the upper plate during Early Cretaceous (‘Austrian’) east-facing orogeny. Turonian to Campanian zircon FT cooling ages (95–71 Ma) from the Bihor and Codru nappe systems and the Biharia and Baia de Arie? nappes (at present the structurally lowest part of the Dacia Mega-Unit) record exhumation that immediately followed a second Cretaceous-age (i.e. Turonian) orogenic event. Thrusting during this overprinting event was NW-facing and led to the overall geometry of the present-day nappe stack in the Apuseni Mountains. Zircon FT ages, combined with thermal modelling of the apatite FT data, show relatively rapid post-tectonic cooling induced by a third shortening pulse during the latest Cretaceous (‘Laramian’ phase), followed by slower cooling across the 120°–60 °C temperature interval during latest Cretaceous to earliest Paleogene times (75–60 Ma). Cenozoic-age slow cooling (60–40 Ma) was probably related to erosional denudation postdating ‘Laramian’ large-scale updoming.     

Single and double exhumation of fault blocks in the internal Sesia-Lanzo Zone and the Ivrea-Verbano Zone (Biella, Italy)

The combination of magmatic, structural and fission track (FT) data is used to unravel Oligocene/Miocene near-surface tectonics in the internal Western Alps. This includes reburial of parts of the already exhumed Sesia-Lanzo Zone and their subsequent re-exhumation. We define blocks mainly on the base of their Oligocene–Miocene cooling history (FT data) and on published paleomagnetic data. The preservation of a paleosurface allows a detailed reconstruction of the exhumation, burial and re-exhumation of different tectonic blocks. Near-surface, rigid block rotation is responsible for the reburial of the Lower Oligocene paleosurface in part of the Sesia-Lanzo Zone (the Cervo Block) and for the conjugate uplift of deeper portions of the Ivrea-Verbano Zone (the Sessera-Ossola Block). This block rotation around the same horizontal axes produces in the currently exposed portions of the two blocks, quite different temperature/time paths. While the surface of the Cervo Block is buried, the lower part of the Sessera-Ossola Block is uplifted. The rotation is constrained between the age of emplacement of the Biella Volcanic Suite on top of the Sesia-Lanzo Zone (32.5?Ma) and the intrusion of the Valle del Cervo Pluton (30.5?Ma). After this relative fast movements, the concerned blocks remained in (or underneath) the partial annealing zone of zircon until in Aquitanian times they were rapidly uplifted into the partial annealing zone of apatite. The further stage of exhumation out of the partial annealing zone of apatite extends over the entire Miocene. At that time, units of the external Western Alps underwent fast exhumation (external Brian?onnais, Valais). In addition to the well-known post-collisional deformation in the axial- and external Western Alps, the internal units (i.e., the upper plate) hold an apparent stable position in terms of exhumation.     

裂变径迹定年方法的进展及应用

裂变径迹分析方法利用裂变径迹年龄及裂变径迹长度分布形态来反映随时间产生的裂变径迹积累及由温度控制的退火(裂变径迹缩短或消失)两种因素综合作用的结果。近几年来，有关磷灰石的退火特征进行了大量实验和野外研究，取得了较大进展。本文通过对80年代以来国内外裂变径迹研究状况的追踪调研结果，简要介绍裂变径迹定年方法的主要进展及应用情况。     

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.     

Thermal history of granitic rocks from western Victoria: A fission‐track dating study

Fission‐track ages have been determined on sphene and apatite from 28 granitic intrusions across the western half of Victoria. The sphene ages compare closely with independent K‐Ar biotite ages for the same intrusions, where these are available, and are invariably older than apatite ages by 35 to 135 m.y. This is in accord with the effective geological track annealing temperatures for these two minerals which are estimated to be 260 ± 20°C and 80 ± 10°C respectively. Both sphene and apatite ages decrease from west to east across western Victoria, the sphenes ranging from 470 ± 28 to 355 ± 19 m.y. The Wando Vale granodiorite and Dergholm granite from the Dundas Tableland of far‐western Victoria have sphene ages of 470 ± 28 m.y. and 452 ±16 m.y. respectively, clearly suggesting a relationship to the Ordo‐vician granitic rocks of southeastern South Australia. Fission‐track ages from the numerous post‐tectonic granites in the Ballarat Trough fall into two distinct groups. Rocks from the western area have sphene ages in the relatively narrow range 393 ± 14 m.y. suggesting emplacement in the Early Devonian time whereas those in the east have sphene ages of 362 ± 7 m.y. (near the Devonian‐Carboniferous boundary). Over the temperature interval recorded by sphene‐apatite pairs, cooling of the granitic rocks was very slow ranging from 0.8 to 5.3°C/m.y. Cooling in this range was probably controlled by uplift and erosion of overburden during a long period of post‐tectonic relaxation. Corresponding uplift rates are estimated to be 0.03 to 0.18 km/ m.y. assuming a normal continental geothermal gradient of 30°C/km. Below 80°C average cooling and uplift rates were probably about l°C/m.y. and 0.03 km/m.y. respectively so that cooling was essentially complete within about 80 m.y. of the apatite ages.     

Regional thermal history of the Lennard shelf,Canning Basin,from apatite fission track analysis: Implications for the formation of Pb‐Zn ore deposits

Zinc mineralization in Devonian carbonates of the Lennard Shelf, northern Canning Basin is similar in many respects to that of the Mississippi Valley‐type including estimated minimum temperatures of sulphide precipitation between 70 and 110°C. Apparent apatite fission track ages for Precambrian granitic basement and for detrital apatites in Devonian carbonates in and near Pb‐Zn mineralization generally range between 260 and 340 Ma, with Precambrian samples tending to have slightly older apatite fission track ages than the Devonian carbonates. These apparent ages are younger than the stratigraphic age of the material analysed, indicating that appreciable annealing of fission tracks in apatite has occurred in post‐Devonian times. Mean horizontal confined track lengths are 12–13 μm for most samples and preclude attaching any ‘event’ significance to the fission track ages. Studies of well sequences (Grevillea 1 and Kennedia 1) indicate a period of rapid uplift in the area during the Late Triassic/Early Jurassic. Assuming a constant geothermal gradient of 30°C/km, approximately 1.5 km of uplift and erosion is estimated. Immediate thermal effects related to Miocene lamproite intrusion into Precambrian basement appear to be restricted to within 200 m of the contact zone.

For outcropping Devonian carbonates, a thermal history is proposed involving burial in the Late Palaeozoic/Early Mesozoic, followed by uplift and cooling from peak temperatures around 70–80°C in mid‐Mesozoic times. With reference to this period of burial, Pb‐Zn occurrences represent thermal anomalies when reported fluid inclusion homogenization temperatures are compared with the estimated peak temperatures. However the possibility of a phase of higher temperatures during the Late Devonian/ Early Carboniferous is suggested by the apatite fission track results, in which case sulphide mineralization may reflect ambient regional temperatures if it formed at that time. The absence of enhanced annealing effects in detrital apatites proximal to Pb‐Zn deposits suggests that either sulphide mineralization preceded or accompanied peak regional temperatures suspected during the Late Devonian/Early Carboniferous, or that the mineralizing episodes were of too short a duration to significantly anneal fission tracks in apatite.