Cooling history of the Sanbagawa metamorphic belt inferred from fission track zircon ages

Zircon and apatite fission track (FT) reveal some of the thermo-tee- tonic features of the Marlborough Region, South Island, New Zealand. The very young FT ages (＜10 Ma) of zircon and apatite in the vicinity of the Alpine Fault bend and Seaward Kaikoura Range coincide with the recent rapid uplift/erosion. Four samples with reset zircon ages in the Alpine Fault bend reveal that the host rocks in this area cooled below the closure termperature of zircon (~240℃) in the late Miocene. Unlike these four zircon FT ages, most zircon FT ages are consistent with depositional ages. Annealed apatite and unannealed zircon FT ages show that the Marlborough did not experience exposure to the closure temperature of zone (PAZ) of apatite (~60-100℃). The host rocks in the north rather than those in the south passed through the lower part of apatite PAZ. In addition, most of the zircon samples with low P( X2) values (＜5 %) show that the samples have been slightly annealed, implying that the host rocks might have experienced the upper part of the partial annealing zone of zircon (~175℃) during the Mesozoic cooling.

The Torlesse Complex comprises several Mesozoic accretionary prism complexes together forming continental basement over large parts of New Zealand. This study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern North Island that crosses the structural grain of the Torlesse Complex, including its older and younger parts. Zircon fission track (FT) ages for the Late Triassic Rakaia Terrane, which is the most inboard of the accretionary complexes, are partially annealed, some possibly reset, and may indicate early Cretaceous (134±10 Ma) cooling from maximum temperatures (Tmax), probably related to imbrication of younger complexes of the Pahau Terrane. Numerical modeling of the zircon FT ages and published 40ArA/39Ar muscovite and biotite ages for the Rakaia Terrane suggest Tmax values of 265–310°C and exhumation from depths of 10–12 km. The rocks underlying the Aorangi Range and involving the youngest accretionary complex have experienced much lower Tmax values of ≤210° and ≥110°C, bracketed by reset apatite FT ages and detrital zircon FT ages. The occurrence of a circa 100 Ma component of zircon FT ages in both the weakly and highly indurated rocks beneath the Aorangi Range, as well as in remnants of an overlying Albian accretionary slope basin (Whatarangi Formation), imply multistorey accretion and incorporation of sediment into the youngest prism. This circa 100 Ma zircon FT age component also places a maximum age on the termination of Mesozoic subduction beneath the New Zealand region. The occurrence of reset apatite FT ages across the whole of the Wellington transect indicates that at least 4 km of exhumation occurred during the late Miocene.

Greywacke sandstone and argillite beds comprising Rakaia Terrane (Torlesse Complex) in mid Canterbury, South Island, New Zealand, are widely regarded as Late Triassic (Norian) in age based on the occurrence of Torlessia trace fossils, Monotis, and other taxa. This paleontological age assignment is tested using published 40Ar/39Ar mica and U‐Pb zircon ages for these rocks and published and new zircon fission track (FT) ages. The youngest U‐Pb zircon ages in the Rakaia Terrane rocks in mid Canterbury are Norian, whereas 10–20% of the 40Ar/ 39Ar muscovite ages are younger than Norian. Numerical modelling of these mica ages shows that they cannot have originated from partial thermal overprinting in the Torlesse prism if the thermal maximum was short‐lived and early in the prism history (210–190 Ma), as commonly inferred for these rocks. The young component of mica ages could, however, be explained by extended residence (200–100 Ma) at 265–290°C in the prism. Early Jurassic (c. 189 Ma) zircon FT ages for sandstone beds from Arthur's Pass, the Rakaia valley, and the Hermitage (Mt Cook) are interpreted not to have experienced maximum temperatures above 210°C, and therefore cannot have been reduced as a result of partial annealing in the Torlesse prism. This is based on identification of a fossil Cretaceous, zircon FT, partial annealing zone in low‐grade schists to the west, and the characteristics of the age data. The Early Jurassic zircon FT ages and the young component of 40Ar/39Ar mica ages are regarded therefore as detrital ages reflecting cooling in the source area, and constrain the maximum depositional age of parts of the Rakaia Terrane in mid Canterbury. The zircon FT data also show the initiation (c. 100 Ma) of marked and widespread Late Cretaceous cooling of Rakaia Terrane throughout Canterbury, which is attributed to uplift and erosion of inboard parts of the Torlesse prism due to continuing subduction accretion at its toe. The critical wedge concept is proposed as a new framework for investigating the development of the Torlesse Complex. The Rakaia Terrane may have formed the core of an accretionary wedge imbricated against the New Zealand margin during the Middle or Late Jurassic. Late Jurassic nonmarine sediments (e.g., Clent Hills Formation) accumulated upon the inner parts of the prism as it enlarged, emerged, and continued to be imbricated. Exhumation of Otago Schist from c. 135 Ma may mark the development of a balance (steady state) between sediments entering the prism at the toe and material exiting at the inboard margin. The enlargement of the area of exhumation to all of Canterbury from c. 100 Ma may reflect a dynamic response to widening of the prism through the accretion of Cretaceous sediments. The model of a dynamic critical wedge may help to explain the various expressions of the Rangitata Orogeny.

Thermobarometric data from a fossil zircon partial annealing zone in high pressure–low temperature rocks of eastern and central Crete, Greece

Comparison of fission-track (FT) ages of detrital zircons recovered from Atlantic Coastal Plain sediments to FT ages of zircons from bedrock in source terranes in the Appalachians provides a key to understanding the provenance of the sediments and, in turn, the erosional and depositional history of the Atlantic passive margin. In Appalachian source terranes, the oldest zircon fission-track (ZFT) ages from bedrock in the western Appalachians (defined for this paper as the Appalachian Plateau, Valley and Ridge, and far western Blue Ridge) are notably older than the oldest ages from bedrock in the eastern Appalachians (Piedmont and main part of the Blue Ridge). The age difference is seen both in ZFT sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks. In the east, ZFT data indicate that the rocks cooled from temperatures high enough to partially or totally reset ZFT ages during the Paleozoic and (or) Mesozoic. The majority of the rocks are interpreted to have cooled through the ZFT closure temperature (∼235 °C) at various times during the late Paleozoic Alleghanian orogeny. In contrast, most of the rocks sampled in the western Appalachians have never been heated to temperatures high enough to totally reset their ZFT ages. Reflecting their contrasting thermotectonic histories, nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old FT ages, in excess of 800 Ma; zircon grains yielding FT ages this old have not been found in rocks in the Piedmont and main part of the Blue Ridge. The ZFT data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western Appalachians was in evidence by no later than the Early Cretaceous and probably by the Late Triassic. Detrital zircon suites from sands collected in the Atlantic Coastal Plain provide a record of detritus eroded from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, sands of Early Cretaceous through late early Oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western Appalachians. Very old zircons yielding FT ages \>800 Ma are only encountered in Coastal Plain sands of middle early Miocene and younger age. Miocene and younger fluvial-deltaic deposits associated with the major mid-Atlantic Coastal Plain rivers that now head in the western Appalachians (the Hudson, Delaware, Susquehanna, Potomac, James, and Roanoke) contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from Paleozoic rocks of the western Appalachians. These distinctive clasts have not been reported in older Coastal Plain sediments. The ZFT and lithic detritus data indicate that the drainage divide for one or more east-flowing mid-Atlantic rivers migrated west into the western Appalachians, and the river(s) began transporting western Appalachian detritus to the Atlantic Coastal Plain, sometime between the late early Oligocene and middle early Miocene. By no later than late middle Miocene most if not all of the major rivers that now head west of the Blue Ridge were transporting western Appalachian detritus to the Coastal Plain. Prior to the drainage divide migrating into the western Appalachians, the ZFT data are consistent with the dominant source of Atlantic Coastal Plain sediments being detritus from the Piedmont and main part of the Blue Ridge, with possible input from distant volcanic sources. The ZFT data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle Atlantic margin offshore basins that peaked in the middle Miocene and produced almost 30 percent of the total volume of post-rift siliciclastic sediments in the offshore basins began in the early Miocene when Atlantic river(s) gained access to the relatively easily eroded Paleozoic sedimentary rocks of the western Appalachians.

피션트랙(fission-track: FT) 연대측정 초기단계의 부적합한 연대보정법에 기인한 오류 원인을 정밀진단하고, 중복시료에 의한 재실험과 제타보정법에 의해 최초 보고된 피션트랙(FT) 연대를 재정의한다. 재검토된 FT 저콘연대는 기반암인 유천층군 유문암질-데사이트질응회암의 생성연대를 후기 백악기부터 고제3기 초(<TEX>$78{\pm}4$</TEX> Ma부터 <TEX>$65{\pm}2$</TEX> Ma)로 재정의하며, 곡강동유문암질응회암을 전기 에오세(<TEX>$52.1{\pm}2.3$</TEX> Ma) 산물로 정의한다. 전기 마이오세 화산암의 경우, 효동리화산암류 상부 데사이트질응회암(<TEX>$21.6{\pm}1.4$</TEX> Ma)과 범곡리화산암류 최상부 데사이트 용암(<TEX>$21.3{\pm}2.0$</TEX> Ma)의 FT 저콘연대는 각각 어일분지 남부와 와읍분지 중앙부의 상부 범곡리층군의 연대층서를, 그리고 금오리데사이트질응회암(<TEX>$19.8{\pm}1.6$</TEX> Ma)의 FT 저콘연대는 장기분지 내 후기 데사이트질 화산활동 시기를 정의한다. 데사이트질암의 FT 저콘연대와 현무암질-안산암질암의 기존 연대자료(대부분 K-Ar 전암, 일부 Ar-Ar)를 기반으로 하여, 한국 동남부의 마이오세 분지의 화산암과 기반암의 층서대비에 길잡이가 될 수 있는 참고연대를 설정 제안한다. 관계화산암의 연대에 기반하여 분지충전 퇴적지층의 퇴적시기도 추정한다. 제안된 참고연대는 마이오세 분지의 복잡한 분지구조와 비교적 짧은 연대범위에도 불구하고 지질층서에 잘 부합된다. 범곡리층군은 어일-와읍분지에서 장기층군보다 하위인 것이 확실하나, 장기분지의 전기 장기층군과 상당부분 중첩 병립되므로, 두 층군을 획일적으로 선후관계로 정의할 수는 없다. 장기분지에서 장기층군 하나로 묶여있는 일련의 지층군은 약 20 Ma를 기준으로 전기(23-20 Ma)의 안산암질-데사이트질암과 후기(20-18 Ma)의 현무암질암으로 뚜렷하게 구분된다. Erroneous fission-track (FT) ages caused by an inappropriate calibration in the initial stage of FT dating were redefined by re-experiments and zeta calibration using duplicate samples. Revised FT zircon ages newly define the formation ages of Yucheon Group rhyolitic-dacitic tuffs as Late Cretaceous to Early Paleocene (<TEX>$78{\pm}4$</TEX> Ma to <TEX>$65{\pm}2$</TEX> Ma) and Gokgangdong rhyolitic tuff as Early Eocene (<TEX>$52.1{\pm}2.3$</TEX> Ma). In case of the Early Miocene volcanics, FT zircon ages from a dacitic tuff of the upper Hyodongri Volcanics (<TEX>$21.6{\pm}1.4$</TEX> Ma) and a dacitic lava of the uppermost Beomgokri Volcanics (<TEX>$21.3{\pm}2.0$</TEX> Ma) define chronostratigraphies of the upper Beomgokri Group, respectively in the southern Eoil Basin and in the Waeup Basin. A FT zircon age (<TEX>$19.8{\pm}1.6$</TEX> Ma) from the Geumori dacitic tuff defines the time of later dacitic eruption in the Janggi Basin. Based on FT zircon ages for dacitic rocks and previous age data (mostly K-Ar whole-rock, partly Ar-Ar) for basaltic-andesitic rocks, reference ages are recommended as guides for stratigraphic correlations of the Miocene volcanics and basements in SE Korea. The times of accumulation of basin-fill sediments are also deduced from ages of related volcanics. Recommended reference ages are well matched to the whole stratigraphic sequences despite complicated basin structures and a relative short time-span. The Beomgokri Group evidently predates the Janggi Group in the Eoil-Waeup basins, while it is placed at an overlapped time-level along with the earlier Janggi Group in the Janggi Basin. Therefore, the two groups cannot be uniformly defined in a sequential order. The Janggi Group of the Janggi Basin can be evidently subdivided by ca. 20 Ma-basis into two parts, i.e., the earlier (23-20 Ma) andesitic-dacitic and later (20-18 Ma) basaltic strata.

K-Ar biotite and fission-track (FT) zircon ages were determined on the Nisatai Dacite (ND), Iwate Prefecture, Japan, to assess its suitability for a geologic age standard. The concentration of potassium and argon isotope ratios were determined on a biotite sample twice and three times, respectively, and yielded reproducible results within analytical errors. A K-Ar age was given as 21.0±0.3 (2 sigma) Ma. FT zircon ages were determined on two samples from different localities as 21.8±1.4 (2 sigma) and 23.9±1.4 Ma. The K-Ar age was obtained from an outcrop adjacent to that for the former FT age, with a good agreement between the two methods. The concordance in age, in conjunction with previous zircon FT length data, suggests a potential applicability of the ND to both the K-Ar and FT system calibration in Cenozoic time.

Tracking crustal processes by FT thermochronology in a forearc high (Hikurangi margin, New Zealand) involving Cretaceous subduction termination and mid-Cenozoic subduction initiation

The Himalayan orogen represents a fold-and-thrust belt characterized by a series of foreland-propagating thrust systems with an out-of-sequence thrust (OST). Geological and geomorphological evidence implies that the Himalayan upheaval started from the north and shifted gradually southward with time. The uplift seems to have been accelerated in the Higher Himalaya since the Pliocene. To investigate the role of OST in recent uplifts of the Higher Himalaya and to estimate the denudation rate of the latter, we carried out fission track (FT) dating on zircon and apatite of high-grade pelitic and granitic gneisses from both sides of an inferred OST north of the Kathmandu nappe and along a traverse across Gosainkund Lekh in the northern root zone of the Kathmandu nappe.Nine zircon FT ages of sites in the Gosainkund Lekh area range from 1.5 Ma (at 1525 m) to 2.8 Ma (at 5045 m). Ages of four samples from the northern slope of the area increase linearly with elevation, and yield an average denudation rate of 2.4 mm/y. On the other hand, six samples from the southern slope yield identical ages (2.5-2.8 Ma) regardless of their sample elevations (1150-5045 m). The zircon ages of the area suggest that rocks on the southern slope passed evenly the depth of closure temperature of zircon at around 2.6 Ma. Following this, the northern part of the southern slope (the highest part of Gosainkund Lekh) and the northern slope were uplifted more rapidly than the southern slope.Eight zircon FT ages and seven apatite FT ages from rocks around the Sheopuri zone range from 4.4 Ma to 6.9 Ma and 4.3 Ma to 5.5 Ma, respectively. These ages vary irrespective of their sample elevations (1200-2400 m). Furthermore, zircons and apatites from both sides of the OST (Trisuli-Likhu fault) yield almost the same FT ages, suggesting that the OST has been inactive since the latest Miocene. It is noteworthy that zircon FT ages from the sample elevations between 1200 m and 1800 m in the present area together with zircon FT ages (ca. 9 Ma) reported from early Paleozoic granites in the southern part of the Kathmandu nappe suggest a general trend of younging towards the north between 9 Ma and 1.5 Ma. On the basis of such a trend, two possible uplift mechanisms of the Kathmandu nappe and its northern root zone in the Higher Himalaya are presented. Both mechanisms might be related to crustal ramp due to northward wedging of Indian mid-crust beneath the Himalaya.

New apatite and zircon fission track and (U‐Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7 zircon fission track (ZFT), and 7 apatite (U‐Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie ∼2 km above the adjacent valleys, and a transect of &gt;20 valley bottom samples spans from the Lesser Himalaya across the Greater Himalaya and into the Tethyan strata. As a consequence, these data provide one of the more comprehensive low‐temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFT ages range from &gt;3.8 to 0 Ma, ZFT ages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the south central zone of the field area) yielded significantly older ZFT and AFT ages of ∼2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFT and AFT ages from a vertical transect along this ridge indicate exhumation rates of ∼1.5 km Myr−1(r2&gt; 0.7) from ∼2 to 0.6–0.8 Ma, whereas AHe ages indicate a faster exhumation rate of ∼2.6 km Myr−1(r2= 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr−1(all with low r2values of &lt;0.6) for the time period from ∼1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long‐term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than in the Greater Himalaya and may be a response to a drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data are further evaluated with thermokinematic modeling in the companion paper by Whipp et al.

Fission track (FT) analyses on unannealed detrital min- erals provide a powerful tool both for refining provenance models de- rived from traditional methods and for collecting information about erosion rates of the source area. Their power is increased if they are coupled with the study of zircon morphology. This combination of methods is applied to the Chattian-Aquitanian (25-23 Ma) Macigno turbidite complex. Basin-fill patterns and petrographical studies con- sistently identify the uplifting western Central Alps as the main source region for the Macigno Formation. Most zircon grains fall into a young age cluster ( ; 40-30 Ma), de- rived from a rapidly exhuming crystalline source region with a high cooling rate. Within this cluster, two age subgroups can be distin- guished at 30 and 40 Ma. In the younger subgroup, the zircon mor- phology supports the presence of two main populations: (1) from ig- neous rocks (S-type euhedral zircons), which appear to be partly de- rived from airborne tuffs; and (2) from metasedimentary units. In huge volumes of these metamorphic rocks, mica Ar-Ar and zircon fission- track thermochronometers have been reset, because of high geothermal gradients in the vicinity of the Periadriatic intrusives in mid-Oligocene times. At the present surface of the Alps, zircon FT ages around and slightly less than 30 Ma are reported in the Sesia-Lanzo zone, the Gran Paradiso Massif, the Upper Pennine nappes, the Monte Rosa Massif, and the Dent Blanche complex. The older subgroup of the Tertiary zircons (40 Ma) may have been supplied by metamorphic and mig- matitic rocks affected by an Eocene high-temperature phase. A Late Cretaceous age cluster ( ; 70-60 Ma) is related to cooling after the main Austroalpine metamorphic event at 110-100 Ma. Most of the recently exposed Austroalpine nappe complex displays mica cooling ages and zircon FT ages between 95-70 Ma and 99-55 Ma, respectively. Finally, an ill-defined Jurassic age cluster, with a mean in Late Ju- rassic times, is related to rift-shoulder heating of the Austroalpine/ South-Alpine crystalline basement due to rifting of the Pennine oceanic domain. Presently, the Silvretta nappe complex, situated at the western termination of the Austroalpine realm, and the South-Alpine basement west of the Canavese Line, display similar zircon FT ages. Therefore, a westward continuation of the Silvretta complex prior to deep Neo- gene erosion is suggested.

We analyzed the fission-track (FT) thermochronology of zircon in two samples of psammitic schist from the chlorite zone of the Sambagawa metamorphic belt, central Shikoku, Japan. The samples were collected from the lower-grade part of the chlorite zone (pumpellyite-actinolite facies). Detrital zircons from a single locality along the Asemi River are completely annealed, yielding FT ages of 92.6±6.2 Ma (1σ, 13 grains), and those from a single locality along the Dozan River are also completely annealed, yielding FT ages of 47.2±3.8 Ma (1σ, 7 grains). Although both samples were collected from within the chlorite zone, the two FT zircon ages are different, possibly because they represent the timing of peak metamorphism in the former sample, and a D3 thermal event (post peak-metamorphism) in the latter sample.

Fission-track (FT) dating of pseudotachylyte (PST) associated with the Median Tectonic Line (MTL) in the Taki area, Mie Prefecture, SW Japan, provides new constraints on the timing of movement upon this fault. A PST vein with a thickness of about 5 cm yields a zircon FT age of 60.0 ± 3.5 Ma (1σ). In contrast, a weighted average of zircon FT ages obtained for protolith samples (cataclastic mylonitized Hatai Tonalite) collected 10 cm and 15 m from the PST vein boundary is 69.8 ± 1.2 Ma, which is significantly older than the age of the PST. Decomposition of feldspars in the PST suggests that the temperature exceeded 1100°C, which is sufficient to completely erase previous fission tracks in zircon within several seconds. The distribution of fission-track lengths in zircon from the PST vein also supports the interpretation that the zircon FT age was completely reset during frictional fusion of the PST vein. Considering an apatite FT age of 38.0 ± 1.5 Ma for the host rock, the age of the PST indicates that the frictional fusion was occurred during cooling of the Ryoke granite at temperatures between 250°C (closure temperature of the zircon FT system) and 100°C (closure temperature of the apatite FT system). This PST age is comparable with the oldest K-Ar age obtained for fine fractions of MTL fault gouge derived from both the Sanbagawa pelitic schist and the Izumi Group in Shikoku, indicating that the initiation of brittle fault movement associated with formation of the PST and/or fault gouges along the MTL had occurred by 60 Ma.

Thermochronologic constraints on evolution of the Linglong Metamorphic Core Complex and implications for gold mineralization: A case study from the Xiadian gold deposit, Jiaodong Peninsula, eastern China

Fission track dates of Mandi granite and adjacent tectonic units in Kulu–Beas valley, NW Himalaya, India