Zircon Fission Track Thermochronology Research Articles

Detrital thermochronology and sediment petrology of the middle Siwaliks along the Muksar Khola section in eastern Nepal

Sediment derived from erosion of the Himalaya during the Miocene–Pliocene was deposited in the Himalayan foreland basin and is now exposed in the Siwalik Formation between the Main Boundary Thrust and Main Frontal Thrust. These sediments hold important information on orogenic exhumation during this time. In this preliminary study we sampled the middle Siwalik Formation in eastern Nepal along the Muksar Khola section for thermochronologic and sediment petrologic analysis. Detrital zircon fission-track thermochronology shows that the age signal of the middle Siwalik Formation of eastern Nepal has some similarities but also differences with published data for western-central Nepal. The Muksar Khola section samples show two static peaks at about 14 and >140 Ma, which is similar to the Siwalik Formation of western-central Nepal. In contrast, a signal of consistently fast exhumation was not observed in the upper middle Siwaliks in eastern Nepal. This could be related to a position of the Main Central Thrust further south, to overall less erosional exhumation of Higher Himalayan rocks, or to a reduced exposure of the underlying Lesser Himalayan units in comparison with western-central Nepal. Based on our apatite fission-track data we propose that above the section dated by Ojha et al. (2009) with magnetostratigraphy about 1000 m of overlying upper Siwalik Formation exist.

Zircon fission track thermochronology constraints on mineralization epochs in Altai Mountains, northern Xinjiang, China

The mineralization ages reported in the past in the Altai Mountains, northern Xinjiang, China, not only are different, but also fall into the Hercynian epoch. This study has achieved 17 zircon fission track analysis results from different ore deposits. The zircon fission track ages range from 134 ± 9 Ma to 339 ± 49 Ma. We consider that the zircon fission track age could reveal the mineralization age based on an annealing zone temperature of 140–300 °C and a retention temperature of ∼250 °C for zircon fission tracks, and a metallogenetic temperature of 150–260 °C in various ore districts. The mineralization age agrees with the regional tectonics. A total of four mineralization epochs have been identified, i.e., 339–312 Ma, 255–234 Ma, 218–174 Ma and 158–126 Ma, and these indicate occurrence of the mineralizations in the Indosinian and Yanshan epochs as well as the Hercynian epoch.

New thermochronologic constraints on the evolution of the Zaldívar porphyry copper deposit, Northern Chile

Life spans and thermal evolution of hydrothermal systems are of fundamental metallogenic importance. We were able to establish the chronology and cooling history of the Zaldivar porphyry copper deposit (Northern Chile) by applying a combination of different isotopic dating methods in minerals with different closure temperatures, including 40Ar/39Ar geochronology and zircon fission track thermochronology, together with fluid inclusion thermometry and previous published U–Pb zircon geochronology. The hydrothermal mineralization in the Zaldivar deposit is genetically related to the Llamo Porphyry unit. Samples of igneous biotites from this intrusion yielded 40Ar/39Ar plateau ages between 35.5 ± 0.7 and 37.7 ± 0.4 Ma defining a weighted average of 36.6 ± 0.5 Ma (2σ). In contrast, one sample from the Zaldivar porphyry, one from the andesites, and two from the Llamo porphyry yielded considerably younger fission track ages of approximately 29 Ma with a weighted mean for all ages of 29.1 ± 1.7 Ma (2σ). Thermal and compositional constraints for the hydrothermal system in the Zaldivar deposit from fluid inclusions thermometry show that at least three fluid types broadly characterize two main hydrothermal episodes during the evolution of the deposit. The main mineralization and alteration event is characterized by high temperature (above 320°C) hypersaline fluids (salinity between 30 and 56 wt.% NaCl equivalents) coexisting with low-density gas-rich inclusions (salinity less than 17 wt.% NaCl equivalents) that homogenizing into the gas phase at temperatures above 350°C. The second episode corresponds to a low-temperature event which is characterized by liquid-rich fluid inclusions that homogenize into the liquid phase at temperatures ranging from 200°C to 300°C with salinities lower than 10 wt.% NaCl equivalents. The 40Ar/39Ar data (36.6 ± 0.5 Ma, weighted average) obtained from igneous biotites represent the minimum age for the last high-temperature (above 300°C) hydrothermal pulse. When compared with previously published U–Pb ages (38.7 ± 1.3 Ma) in zircons from the Llamo porphyry, a close temporal relationship between crystallization of the parental intrusion and the thermal collapse of the last high-temperature hydrothermal event is evident. Cooling took place from approximately 800°C (crystallization of the intrusive complex defined by zircon U–Pb ages) to below 300 ± 50°C (biotite 40Ar/39Ar closure temperature) within approximately 1.5 m.y. Because the thermal annealing of fission tracks in zircons occurs at temperatures of 240 ± 30°, the zircon fission track (ZFT) ages of 29.1 ± 1.7 Ma (2σ) mark the end of the thermal activity in the Zaldivar area, specifically the time when the whole area cooled below this temperature, well after the collapse of the main hydrothermal event in the Zaldivar porphyry copper deposit. This cooling age roughly coincides with the age defined for the emplacement of dacitic dikes at 31 ± 2.8 Ma (2σ) (published K–Ar whole rock), 5 km south of the Zaldivar deposit, in the Escondida area. This late magmatic pulse probably is responsible for high heat flow in the Zaldivar deposit as late as 29 Ma. There is no evidence that the low temperature hydrothermal pulse recognized by fluid inclusion studies is related to this thermal event. The zircon fission track cooling ages are interpreted to be related to the time lag required for complete relaxation of the perturbation of the isotherms in the geothermal field imposed by the intrusion of magmatic bodies, with or without any association with low temperature hydrothermal activity.

Temperature and age constraints on the metamorphism of the Tethyan Himalaya in Central Nepal: A multidisciplinary approach

Metasediments of Devonian to Triassic age of the Tethyan Himalaya (TH) from several areas in central and western Nepal, between western Dolpo and Marsyandi Valley, were sampled for thermo-metamorphic studies (illite and chlorite “crystallinity”, vitrinite reflectance, calcite–dolomite and chlorite-chloritoid geothermometers, K/Ar dating on illite-rich fractions and zircon fission track thermochronology). This paper reports, for the first time, the occurrence of chloritoid in the TH out of the range of granite contact aureoles. It also presents the first zircon fission track dating performed on TH metasediments. The peak temperatures of metamorphism have been around 250–300 °C, 320–350 °C, 330–370 °C and 400–450 °C in the western Dolpo, Hidden Valley, Manang and Marpha areas, respectively. In the Manang and Hidden Valley areas, illite K/Ar data are interpreted as ages of recrystallized K-white micas newly formed during metamorphism at around 25–30 Ma. In the Marpha area, illite K/Ar and zircon fission-track ages (12–15 Ma) are consistent representing cooling ages after metamorphic overprinting of higher grade than in the other areas. The joint investigation of the organic maturation and zircon FT chronology yields insight on the thermal calibration of zircon reset. The R max of 5.7% to ∼8.0% indicates a temperature range of ca. 315–325 °C which is in the partial annealing zone of the zircon FT thermochronometer. Our results might be explained by the presence of a thick thrust sheet once existing above the study area.

Dating pseudotachylyte of the Asuke Shear Zone using zircon fission-track and U–Pb methods

Zircon fission-track (FT) and U–Pb analyses were performed on zircon extracted from a pseudotachylyte zone and surrounding rocks of the Asuke Shear Zone (ASZ), Aichi Prefecture, Japan. The U–Pb ages of all four samples are ∼ 67–76 Ma, which is interpreted as the formation age of Ryoke granitic rocks along the ASZ. The mean zircon FT age of host rock is 73 ± 7 (2 σ) Ma, suggesting a time of initial cooling through the zircon closure temperature. The pseudotachylyte zone however, yielded a zircon FT age of 53 ± 9 (2 σ) Ma, statistically different from the age of the host rock. Zircon FTs showed reduced mean lengths and intermediate ages for samples adjacent to the pseudotachylyte zone. Coupled with the new zircon U–Pb ages and previous heat conduction modeling, the present FT data are best interpreted as reflecting paleothermal effects of the frictional heating of the fault. The age for the pseudotachylyte coincides with the change in direction of rotation of the Pacific plate from NW to N which can be considered to initialize the NNE–SSW trending sinistral–extensional ASZ before the Miocene clockwise rotation of SW Japan. The present study demonstrates that a history of fault motions in seismically active regions can be reconstructed by dating pseudotachylytes using zircon FT thermochronology.

Meso-Cenozoic uplifting and exhumation on Yunkaidashan: Evidence from fission track thermochronology

Yunkaidashan, located at the southern South China block (SCB) and closely adjacent to the Indochina block, is an ideal region for better understanding the temporal and spatial framework of tectonothermal overprinting at the southern SCB since Mesozoic. Apatite and zircon fission track thermochronology of various-type rocks from Yunkaidashan is presented in this paper. The results show, no matter what rocks are, the apparent ages of zircon fission track range from 97.4 to 133.0 Ma, and those of apatite fission track from 43.2 to 68.4 Ma. The length of apatite fission track yields an average confined track length of ∼13 μm and shows normal distribution of unimodal frequency. It is inferred that the uplifting amplitude has been more than 5 km in Yunkaidashan since late Mesozoic. The difference of fission track apparent ages at different locations in Yunkaidashan suggests a paleophysiognomic scenario of the heterogeneous uplift/denudation. These data of the fission track thermochronology provide new constraints for better understanding the tectonophysiognomic pattern of the SCB since late Mesozoic.

Exhumation History of Southern Madagascar as Revealed by Zircon and Apatite Fission-Track Thermochronology

Exhumation History of Southern Madagascar as Revealed by Zircon and Apatite Fission-Track Thermochronology

Tectonically versus climatically driven Cenozoic exhumation of the Eurasian plate margin, Svalbard: Fission track analyses

Apatite and zircon fission track (FT) thermochronology, as well as regional geologic data, are used to assess the tectonic and climatic mechanisms responsible for exhumation of Svalbard during the last 70 million years. Four stratigraphic profiles in the Paleogene Central basin spanning ∼0.6–1.1 km vertical relief yield apatite FT ages ranging from 28.3 ± 2.3 Ma to 41.8 ± 3.7 Ma, with a mean age of ∼36 Ma and no apparent vertical trend. Apatite ages from the Paleogene Forlandsundet and Kongsfjorden basins and the Late Cretaceous‐Tertiary Svalbard Orogen range from 26.8 ± 3.7 to 55.7 ± 4.7 Ma, with Kongsfjorden data suggesting cooling earlier than the Central basin. Although 10 detrital zircon samples were analyzed from both the Forlandsundet and Central basins, only one sample (from the basal Paleogene deposits in the northwesternmost Forlandsundet basin) had a reset age of 42.4 ± 1.8 Ma. Nonreset detrital zircons elsewhere retain Caledonian and Proterozoic detrital ages. Thermal‐history models derived from apatite track‐length distributions of six Central and Kongsfjorden basin samples indicate a strikingly consistent five‐phase thermal history. Initial uplift of sediment sources to the north and west is recorded as a 70–50 Ma cooling signature. Following deposition from 63 to 49 Ma, the samples were heated until ∼35 Ma, probably as the result of continued sediment burial. This basin filling coincided with a dextral continental transform connecting spreading ridges in the Arctic and Norwegian‐Greenland oceanic basins. The thermal models are consistent with ∼40°–50°C of cooling from ∼35 to 25 Ma. This cooling, which is consistent with deposition patterns both onshore and offshore, appears to be the result of rift‐related uplift and erosion as the Atlantic spreading ridge propagated northward to separate Greenland from Svalbard. Track‐length models are consistent with thermal stasis from ∼25 Ma to 5 Ma, despite sedimentation in offshore Miocene basins suggesting that continued erosion was occurring. We suggest that an increase in the geothermal gradient (by the “Yermak Hot Spot” and associated onshore volcanism) offset cooling expected at this time as the result of continued erosion. A final rapid post‐5 Ma cooling phase of ∼70°C is attributed to ∼2.1 km of erosion in response to intensified glacial denudation. Pliocene‐Holocene cooling, which occurred despite a persistently elevated geothermal gradient, is consistent with offshore evidence for first ice cover in the Norwegian‐Greenland Sea at 5.5 Ma and a substantially increased sediment flux to continental‐slope basins from 5.0 to 0.44 Ma.

Provenance and thermal history of the Franciscan accretionary complex: Constraints from zircon fission track thermochronology

The Franciscan accretionary complex of California contains large belts of sediments that were subducted to depths of ≈10 to 30 km, metamorphosed under high‐pressure and low‐temperature conditions, and later returned to the surface. Reconnaissance zircon fission track (FT) data (21 samples) place constraints on the sedimentary provenance, thermal history, and/or exhumation history of various Franciscan units. In a transect across the northern Coast Ranges through Mendocino Pass, metagraywacke samples from the Valantine Springs Formation of the Franciscan Eastern belt experienced maximum temperatures (Tmax) ranging from ≈250° to >≈310°C. Zircon FT and previously reported apatite FT data indicate that parts of the western Eastern and eastern Central belts cooled at about 100–70 Ma from ≈300°C to ≈100°C. Considering the low paleogeothermal gradient of roughly 9°C/km during Franciscan subduction, these rocks were apparently uplifted and exhumed from ≈30 to ≈10 km depth at this time. One sample from the Eyler Mountain sequence in the northwestern Diablo Range similarly shows Tmax ≥ ≈310°C and major cooling at ≈100–70 Ma. This uplift and exhumation may have reflected accelerated accretion at the base as well as the front of the Franciscan wedge accompanied by extensional thinning at shallower levels, as a result of massive sediment influx into the Franciscan trench from the Sierra Nevada magmatic arc. In the northern Coast Ranges, the western part of the Central belt and the Coastal belt experienced lower Tmax, probably <200°C, and retain source area (detrital) zircon FT ages. Burnt Hills terrane samples in the Diablo Range also experienced Tmax <≈230°C and retain detrital ages. The detrital single‐grain age patterns from these areas are compatible with those expected from the Sierra Nevada magmatic arc and its country rocks. This is consistent with derivation of Franciscan sediments from arc terranes in California and provides no particular suggestion that these parts of the Franciscan are highly allochthonous. Paleogene Coastal belt sandstones also yield detrital ages compatible with derivation from the Sierra Nevada but also include early Tertiary zircon populations compatible with reconstructions suggesting these sandstones were sourced partly from the Idaho Batholith.

Towards zircon fission-track thermochronology: Reference framework for confined track length measurements

Horizontal confined track (HCT) lengths and angles to the crystallographic c-axis were measured for both unannealed spontaneous and induced fission tracks (FT) in zircons to establish a reference framework of HCT length measurement. Induced track lengths were analysed on eleven zircon separates from various rock types, including four age standards, and spontaneous lengths on three zircon standards were analysed. In general, the observed HCT lengths in each sample are slightly anisotropic, with shorter tracks having greater angles to the c-axis. In addition, all samples show large anisotropy in the number of observed HCT's, particularly for induced tracks. As an observed mean HCT length is controlled by the combined influence of these two anisotropies, selective measurement of tracks that have specified azimuth angles to the c-axis (e.g., 60–90°) is recommended in annealing studies or thermochronological applications. Mean spontaneous and induced HCT lengths of 60–90° are approximately concordant for individual age standards, although slightly controlled by the difference in overetching effects due to the different etching criteria adopted. This indicates that natural track annealing at an ambient surface temperature at geological timescales is negligible in zircon, in contrast to the case in apatite. Mean lengths of induced HCT's do not differ significantly between the zircons analysed. Therefore the overall mean and standard deviation values of HCT lengths for 60–90° azimuth angles, i.e. 10.50 ± 0.04 (1 standard error) μm and 0.76 ± 0.03 μm for spontaneous tracks and 10.73 ± 0.04 and 0.63 ± 0.03 μm for induced tracks, respectively, offer reliable references for HCT length measurement in zircon FT thermochronology.