Amphibole Ages Research Articles

Thermochronological constraints on the late Paleozoic tectonic evolution of the southern Chinese Altai

The Chinese Altai, as an accretionary orogen developing from the Cambrian to Carboniferous, was modified by Permian deformation, metamorphism and magmatism in response to the collision with the East Junggar. The tectonic processes of the collision and the timing of related deformation and metamorphism are still enigmatic. Here we present new 40Ar/39Ar dating results for granitic gneisses and amphibolites exposed in the Qiongkuer Domain in two areas (Alahake and Fuyun) of the southern Chinese Altai. The amphiboles from both Aalaheke and Fuyun areas yield consistent 40Ar/39Ar plateau ages at 265.9±1.7Ma and 270.1±3.1Ma. In contrast, the biotites from two areas show distinct 40Ar/39Ar ages of 232.0±2.1Ma and 226.8±1.7Ma for the Alahake area, and 245.1±1.5Ma and 264.5±2.2Ma for the Fuyun area, respectively. Given that 40Ar/39Ar ages are younger than the Permian high temperature metamorphism at ∼299–277Ma as constrained by metamorphic zircons in the Qiongkuer Domain, we interpret these 40Ar/39Ar ages to record the cooling history of two areas. Compatible amphibole ages in both areas indicate a similar exhumation age along the Qiongkuer Domain cooling through 550°C at ∼270–265Ma, which may have been associated with the development of the sinistral Erqis Shear Zone during the collision between the Chinese Altai and the East Junggar. The subsequent exhumation processes were variable in different areas of the Qiongkuer Domain as showed by the distinct biotite ages, which may indicate along-strike variation of cooling processes from 550°C to 300°C in the Qiongkuer Domain.

Geochronology and isotope geochemistry of Eocene dykes intruding the Ladakh Batholith

In order to determine the extent and timing of dyke formation related to possible E–W extension along the southern margin of Eurasia during Early Cenozoic time, we examined ca. 30 mostly andesitic dykes intruding the Ladakh batholith from 10 to 50 km west of Leh (NW India). The dykes in the east of the area trend E–NE and those in the west trend N–NW. The difference in orientation is also evident in the petrography and isotopic signatures. The eastern dykes contain corroded quartz xenocrysts and show negative ε0(Nd) and positive ε0(Sr) values, whereas the western dykes do not contain quartz xenocrysts and exhibit positive ε0(Nd) and near-zero ε0(Sr) values. The variability in Sr–Nd isotopes (ε0(Nd) = 3.6 to − 9.6, ε0(Sr) = 0.4 to 143) and the quartz xenocrysts can best be explained by (differing degrees of) crustal assimilation of the parent magma of the dykes. Separated minerals from five dykes were dated by 40Ar–39Ar incremental heating: amphibole ages range between 50 and 54 Ma, and one biotite separate dated both by Rb–Sr and 40Ar–39Ar gave an age of 45 Ma. One dated pseudotachylyte sample attests to brittle faulting at ca. 54 Ma. The combination of structural field evidence with petrographic, isotopic and geochronological analyses demonstrates that the dykes did not form from a single, progressively differentiating magma chamber, despite having formed in the same tectonic setting around the same time, and that processes such as crustal assimilation and magma mixing/mingling, also played a significant role in magma petrogenesis.

Retrograde metamorphism of the eclogite in North Qaidam, western China: Constraints by joint 40Ar/39Ar in vacuo crushing and stepped heating

Two amphiboles and a syn-metamorphic quartz vein from the Yuka terrane, North Qaidam, western China, have been analyzed by joint 40Ar/39Ar crushing in vacuo and stepwise heating techniques. The crushing in vacuo results provide information to directly constrain the timing of fluid activity and the age of amphibolite-facies retrogression. The stepwise heating results could further be used to decipher the thermal history of the UHP rocks. Amphiboles from amphibolites and quartz vein within garnet-amphibolite lens analyzed by in vacuo crushing yield similarly shaped age spectra and exhibit relatively flat age plateaus for the last several steps. The characteristics of gas release patterns and geochronological data testify to the presence of significant excess 40Ar within the fluid inclusions. The age plateaux with weighted mean ages (WMA) ranges from 488 to 476 Ma for amphiboles and 403 Ma for quartz (2σ). These data points constitute amphibole WMA yielding excellent isochrons with isochron ages of 469 and 463 Ma with initial 40Ar/36Ar ratios of 520 and 334, respectively. The isochron ages are interpreted to represent initial amphibolite-facies retrogression. The data points constituting the quartz age plateaux give an isochron age of 405 Ma with initial 40Ar/36Ar ratio of 295, recording a significant aqueous fluid flow episode during the early Devonian. Age spectra obtained by stepwise heating of amphibole residues remaining after crushing experiments are characterized by younger and relatively complex age spectra, which are probably influenced by the combined effects of resetting argon and/or mineral inclusions. Nevertheless, we note that the spectra shapes have features in common: excluding the last two steps, minimum apparent ages are found at temperatures of around 500 °C, corresponding to 319 and 249 Ma, perhaps representing the time of isotopic resetting or resulting from release gas from mineral inclusions of, e.g., biotite or feldspar. Maximum apparent ages are obtained at temperatures of around 800 °C, corresponding to 418 and 413 Ma, which probably reflect mixed ages of radiogenic resetting and original amphibole. These results indicate that the Yuka eclogites and their retrogressed equivalents were overprinted by multiple thermal events in the Silurian and possibly as young as the Triassic.

Geochronology, geochemistry, and petrogenesis of the Maçka subvolcanic intrusions: implications for the Late Cretaceous magmatic and geodynamic evolution of the eastern part of the Sakarya Zone, northeastern Turkey

The Maçka subvolcanic intrusions (MSIs) in the eastern part of the Sakarya zone, northeastern Turkey, play a critical role in understanding the petrogenetic and geodynamic processes that took place during the growth of Late Cretaceous arc crust of this region. U–Pb zircon (79.97 ± 0.97 Ma) and two 40Ar–39Ar amphibole ages (average 81.37 ± 0.5 Ma) indicate that the MSIs were emplaced in Late Cretaceous (Campanian) time into the coeval volcanic rocks. A slightly younger zircon fission track (FT) age (73 ± 9 Ma) points to a rapid exhumation and cooling after crystallization. The intrusions are observed in areas less than 1 km2 in the field and contain abundant mafic microgranular enclaves (MMEs). The host rocks (HRs) are entirely composed of tonalite (SiO2 = 63–65 wt.%, Mg# = 43–52), and the MMEs are gabbro-diorite in composition (SiO2 = 53–57 wt.%, Mg# = 45–48). Both the HRs and the MMEs are I-type, high-K calc-alkaline in composition and display a metaluminous character. They are characterized by geochemical features typical for magmas of subduction-related environments. Chondrite-normalized REE patterns are moderately fractionated [(La/Yb)N = 6–11] and display slightly negative Eu anomalies (Eu/Eu* = 0.7–0.9), with weak concave-upward REE patterns, suggesting that amphibole fractionation played a role during their evolution. The MMEs have slightly different ISr (0.7081–0.7085) and εNd (−5.0 to −5.4) values compared with those of their HRs (ISr = 0.7084–0.7087 and εNd = −5.7 to −6.9), indicating that variable amounts of crustal and mantle components were involved in the generation of parental magma to these rocks. All of these data, combined with those of previous regional studies, suggest that the MSIs are hybrid in origin, produced by the mixing of enriched lithospheric mantle- and lower crust-derived melts in an extensional arc setting that was caused by slab rollback.

Paleozoic HP granulite-facies metamorphism and anatexis in the Dulan area of the North Qaidam UHP terrane, western China: Constraints from petrology, zircon U–Pb and amphibole Ar–Ar geochronology

HP mafic and felsic adakitic granulite bodies in the Dulan area of the North Qaidam ultrahigh-pressure (UHP) terrane record high-pressure (HP) granulite-facies metamorphism and anatexis and provide temporal and tectonic constraints on deep subduction of continental crust and its subsequent exhumation. Mafic HP granulite components dominate the main outcrop and preserve features diagnostic of anatexis and locally may be described as migmatite. The HP mafic granulites comprise garnet, clinopyroxene, plagioclase and quartz. The felsic granulite (leucosome) is mainly composed of K-feldspar+plagioclase+quartz+kyanite+garnet. Detailed zircon U–Pb and amphibole Ar–Ar geochronology, combined with trace element geochemistry, indicate peak metamorphism for the mafic HP granulite at 434±3 to 435±3Ma and peak metamorphism and partial melting for the felsic HP granulite at 433±5 to 438±4Ma, which overlaps the ages of UHP metamorphism for adjacent eclogite (430–446Ma). 40Ar/39Ar amphibole ages of 423 to 432Ma represent amphibolite-facies retrograde metamorphism and indicate rapid cooling during exhumation of the HP granulite bodies. Our geochronological data, combined with field relationships, petrology and geochemistry suggest that HP granulite-facies metamorphism and the partial melting that produced adakitic melts represent the same tectonic event. In this case, the felsic HP granulites (leucosome) formed from an adakitic melt derived from partial melting of mafic HP granulite in the overriding plate in a relatively higher geothermal gradient (15–18°C/km), leaving garnet-cumulate and/or meta-ultramafic (mainly garnet pyroxenite) as the residual component. In contrast, the nearby UHP eclogite is thought to have formed in the subducted plate in a relatively lower geothermal gradient (6–10°C/km). Penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting associated with the North Qaidam continental collisional orogeny during the Late Ordovician–Early Silurian.

Thermal and mechanical behaviour of the orogenic middle crust during the syn‐ to late‐orogenic evolution of the Variscan root zone, Bohemian Massif

AbstractWe combine structural observations, petrological data and 40Ar–39Ar ages for a stack of amphibolite facies metasedimentary units that rims high‐P (HP) granulite facies felsic bodies exposed in the southern Bohemian Massif. The partly migmatitic Varied and Monotonous units, and the underlying Kaplice unit, show a continuity of structures that are also observed in the adjacent Blanský les HP granulite body. They all exhibit an earlier NE−SW striking and steeply NW‐dipping foliation (S3), which is transposed into a moderately NW‐dipping foliation (S4). In both the Varied and Monotonous units, the S3 and S4 foliations are characterized by a Sil–Bt–Pl–Kfs–Qtz–Ilm±Grt assemblage, with occurrences of post‐D4 andalusite, cordierite and muscovite. In the Monotonous unit, minute inclusions of garnet, kyanite, sillimanite and biotite are additionally found in plagioclase from a probable leucosome parallel to S3. The Kaplice unit shows rare staurolite and kyanite relicts, a Sil–Ms–Bt–Pl–Qtz±Grt assemblage associated with S3, retrogressed garnet−staurolite aggregates during the development of S4, and post‐D4 andalusite, cordierite and secondary muscovite. Mineral equilibria modelling for representative samples indicates that the Varied unit records conditions higher than ~7 kbar at 725 °C during the transition from S3 to S4, followed by a P−T decrease from ~5.5 kbar/750 °C to ~4.5 kbar/700 °C. The Monotonous unit shows evidence of partial melting in the S3 fabric at P−T above ~8 kbar at 740–830 °C and a subsequent P−T decrease to 4.5–5 kbar/700 °C. The Kaplice unit preserves an initial medium‐P prograde path associated with the development of S3 reaching peak P−T of ~6.5 kbar/640 °C. The subsequent retrograde path records 4.5 kbar/660 °C during the development of S4. 40Ar–39Ar geochronology shows that amphibole and biotite ages cluster at c. 340 Ma close to the HP granulite, whereas adjacent metasedimentary rocks preserve c. 340 Ma amphibole ages, but biotite and muscovite ages range between c. 318 and c. 300 Ma. The P−T conditions associated with S3 imply an overturned section of the orogenic middle crust. The shared structural evolution indicates that all mid‐crustal units are involved in the large‐scale folding cored by HP granulites. The retrograde P–T paths associated with S4 are interpreted as a result of a ductile thinning of the orogenic crust at a mid‐crustal level. The 40Ar–39Ar ages overlap with U–Pb zircon ages in and around the HP granulite bodies, suggesting a short duration for the ductile thinning event. The post‐ductile thinning late‐orogenic emplacement of the South Bohemian plutonic complex is responsible for a re‐heating of the stacked units, reopening of argon system in mica and a tilting of the S4 foliation to its present‐day orientation.

The origin and age of the metamorphic sole from the Rogozna Mts., Western Vardar Belt: New evidence for the one-ocean model for the Balkan ophiolites

This study brings new geochronological and petrochemical data from the metamorphic sole beneath the Rogozna Mts., Western Vardar ophiolite belt. The Rogozna metamorphic sole is located at the base of a serpentinite nappe and consists of amphibolites and talc-chlorite schists. The Rogozna amphibolites are medium- to fine-grained rocks with nematoblastic texture and pronounced foliation. They consist of green amphibole (~70vol.%) with variable silica contents (6.4 to 7.8Siapfu), as well as Mg# (molMg/[Mg+Fetot]; 0.53 to 0.77) and variably albitized plagioclase (~30vol.%; Ab24–Ab98). Amphibolites are overprinted by a retrograde assemblage containing actinolite, epidote, clinoclore, sericite, chlorite, and magnetite. The amphibolites formed due to metamorphism of two basaltic suites: subalkaline/tholeiitic and alkaline. Subalkaline/tholeiitic amphibolites possess low Zr, Nb, Y, Th, Hf, TiO2, and P2O5 values and a LREE-depleted pattern, typical for the N-MORB (normal mid ocean ridge basalt) to BAB (back-arc basalt) origin. Alkaline amphibolites show elevated concentrations of Zr, Nb, Y, Th, Hf, TiO2, and P2O5 with a LREE-enriched pattern typically displayed by OIB (ocean island basalt). Amphibolites were crystallized during intra-oceanic thrusting at temperatures between 685°C and 765°C and at a depth of 12–17km. 40Ar/39Ar cooling ages of amphibole, ranging from 165 to 170Ma, slightly postdate the sole formation. Talc-chlorite schists are related to retrograde greenschist-facies metamorphism. They consist of Fe-rich talc and Cr-rich chlorite (peninite-diabantite) pseudomorphs after amphibole and MORB-type Cr-Al spinel, surrounded by Al- and Mg-poor ferrit chromite. The occurrence of ferrit chromite is related to earlier, amphibolite facies metamorphism. Chlorite pseudomorphs after amphibole were formed at 300–410°C.This study suggests that there is no essential difference in the emplacement age of the Dinaric and West Vardar ophiolite belts supporting the interpretation involving a single Mesozoic ocean in the Balkan sector.

The 186 Ma Dashibalbar alkaline granitoid pluton in the north-Gobi Rift of central Mongolia: Evidence for melting of Neoproterozoic basement above a plume

The Jurassic Dashibalbar granitoid pluton (300 km 2 ) crops out in the Triassic North-Gobi rift of central Mongolia, just south of the 230 to 195 Ma Khentei batholith. The granitoids are shallow-seated dominantly, amphibole-bearing alkali feldspar granite that contain quartz-syenite/syenite enclaves. They are all composed of megacrystic mesoperthite, quartz, Ca-Na amphibole altered to biotite and rarely with pyroxene cores, magnetite and ilmenite. The pluton yielded a concordant U-Pb zircon age of 186 1 Ma, which is similar to a published 189 3M a 40 Ar/ 39 Ar amphibole age, and indicates rapid cooling through ca. 550 °C. This age is ca. 10 my younger than the 196 4 Ma age of the bimodal volcanic complex intruded by the pluton. The volcanic complex is composed of augite-phyric transitional basalt and rhyolite/ comendite. Both basalts and rhyolites/comendites are evolved within-plate varieties with positive Nd(t) ( 2.5) values. The granitoids are evolved alkaline, A-type granites and quartz-syenites/syenites that are enriched in light REE's, but show a distinct depletion in Eu, Sr and Ba, indicative of feldspar fractionation. The data are consistent with derivation of the granites from the syenites by an assimilation- fractional crystallization process involving a silicic crustal contaminant. The granitic rocks have Nd(t) values of 0.8 to 1.2, which are slightly lower than Nd(t) values 1.3 to 1.6 in the syenites, although both have similar TDM model ages (800-970 Ma). The 800 Ma model ages of the basalt and rhyolite/comendite are comparable to those of the intrusion and enclaves. The compositions of all these rocks, including Nd(t) and T DM , are within the range of A-type granites and volcanic complexes of the Early Mesozoic Mongolian-Transbaikalian igneous province. The results suggest deriva- tion of a parent magma of the granitoids and felsic volcanic rocks from underplated, enriched, Neoproterozoic mantle-derived basaltic rocks in the lower crust, whereas the Dashibalbar basalts were derived from Neoproterozoic subcontinental lithospheric mantle; Neoproterozoic megablocks crop out in adjacent parts of the Central Asian Orogenic Belt. Melting of lower crust and subcontinental lithospheric mantle implies a rising heat source. Although such a heat source is consistent with both rifting and passage over the Mongolian mantle plume, only the latter explains the west-to-east migration of the magmatism and rifting.

Study on the Tectonic Setting for the Ophiolites in Xigaze, Tibet

Abstract:The Xigaze ophiolite is located in the middle section of the Yarlung Zangbo River ophiolite belt and includes a well‐preserved sequence section of seven ophiolite blocks. The relatively complete ophiolitic sequence sections are represented by Jiding, Dejixiang, Baigang, and Dazhuqu ophiolites and consist of three–four units. The complete ophiolite sequence in order from the bottom to top consists of mantle peridotite, cumulates, sheeted sill dike swarms, and basic lavas±radiolarian chert. These cumulates are absent in the remaining blocks of Dejixiang and Luqu. The age of radiolaria in the radiolarian chert is Late Jurassic–Cretaceous. The basalt and ultramafic rock of the ophiolite also are overlaid by Tertiary Liuqu conglomerate, which contains numerous pebble components of ophiolite, indicating that the Tethys Ocean began to close at the end of Cretaceous Period. The isotopic data of gabbro, diabase, and albite granite in the Xigaze ophiolite are approximately 126–139 Ma, which indicates that the ophiolite formed in the Early Cretaceous. The K‐Ar age of amphibole in garnet amphibolite in the ophiolite mélange is 81 Ma, indicating that tectonic ophiolite emplacement occurred at the end of Late Cretaceous. Research in petrology, petrological chemistry, mineralogy, and geochemistry of volcanic rocks and dikes of the Xigaze ophiolite indicate the following characteristics: (1) They are mainly composed of basalt, basaltic andesite, dolerite, and diabase and are characterized by high TiO2 (0.7–1.47%), low MgO (mostly less than 8%), and low SiO2 (mostly less than 53%). (2) The volcanic rocks and dikes of the Xigaze ophiolite show light rare earth element (LREE)‐depleted rare earth element (REE) patterns. (3) The spider diagrams of the volcanic rocks and dikes of the Xigaze ophiolite exhibit LILE depletion relative to high‐field‐strength element (HFSE) patterns with left oblique features. (4) No protogenetic olivine and clinoenstatite was detected. (5) Some dikes show low TiO2 and high MgO, in which a few of Cr‐enriched spinels and a very few pseudomorphs of olivine, orthopyroxene can be seen. They show more distinctive affinity as boninitic rock and canbe classified to boninite series rock. The previously mentioned features of the volcanic rocks and dikes in the Xigaze ophiolite implies that these ophiolites formed in a mid‐ocean ridge (MOR) in the earlier stage and than forearc extension of subduction initiation occurred once at the later stage of the evolution of the Xigaze ophiolite. The forearc extention caused further melting of the residue‐depleted mantle, resulting in the formation of melts with lower TiO2 and higher MgO. These melts formed as dikes and intruded into the oceanic crust formed in the earlier stage, resulting in a close association of mid‐ocean ridge basalt and the boninite rock of the Xigaze ophiolite.

The low-grade Canal de las Montañas Shear Zone and its role in the tectonic emplacement of the Sarmiento Ophiolitic Complex and Late Cretaceous Patagonian Andes orogeny, Chile

The Canal de las Montañas Shear Zone (CMSZ), southern Patagonian Andes (51–52°S), is a low-grade mylonite belt generated from felsic ignimbritic, pelitic and basaltic protoliths of the Late Jurassic–Early Cretaceous Rocas Verdes basin. The different types of rock fabrics across the CMSZ are thought to be associated with relatively intermediate and high strain conditions, characterized by the development of a narrow western belt of S–C´-type mylonites and phyllonites interpreted as the metamorphic sole thrust of the Sarmiento Ophiolitic Complex. Highly strained rocks of the CMSZ display a reverse, continent-ward tectonic transport, with a minor dextral component of shearing. Transitional pumpellyite–actinolite and upper greenschist facies metamorphic conditions at ca. 5–6kbar and 230–260°C indicate that the primary shearing event occurred in a subduction zone setting. In-situ 40Ar/39Ar laserprobe chronology yielded ages of ca. 85Ma on syntectonic phengite which are interpreted as representing cooling synchronous with mica crystallization during the main compressive deformational event. The 78–81Ma U–Pb zircon crystallization ages of cross-cutting plutonic and hypabyssal rocks and 40Ar/39Ar amphibole age of ca.79Ma from lamprophyric dikes within the fold-thrust belt constrain an upper age limit of the ophiolite tectonic emplacement deformation.

A Carboniferous high-pressure klippe in the western Acatlán Complex of southern México: implications for the tectonothermal development and palaeogeography of Pangea

High-pressure (HP) rocks are critical for palinspastic restorations because they mark inferred subducted/extruded oceanic crust; knowledge of their geometric, geodynamic, and age relationships provide essential constraints on palaeogeographic reconstructions. The westernmost HP belt (Ixcamilpa) in the Acatlán Complex of southern Mexico has been inferred to be a mid-Late Ordovician backarc basin on the southern Iapetan margin that was subducted beneath eastern Laurentia and extruded up the subduction zone during the Early Silurian. Re-examination of Ixcamilpa HP rocks has revealed that they comprise lower Palaeozoic rift-passive margin protoliths and occur in a W-vergent klippe (not a suture) formed during polyphase deformation. Peak metamorphic mineral assemblages of blueschist-amphibole eclogite facies underwent retrogression through epidote amphibolite to greenschist facies. 40Ar/39Ar dating of various rocks yielded plateau ages of 344–339 Ma for calcic amphibole, 318 ± 4 Ma for glaucophane, and 329–325 ± 2 Ma for muscovite (excess argon), which clearly indicate a Carboniferous tectonothermal event. We interpret the 20 million years range in amphibole ages as reflecting progressive unroofing. The terminal stage of progressive thrusting placed the HP rocks above the middle Mississippian Zumpango Unit, during which a single penetrative sub-greenschist fabric was produced. Subsequent Permian or Laramide deformation refolded all the rocks about NE-trending upright folds. We postulate that the root zone of the HP nappe lies to the east in the median HP belt, which has a structure consistent with an extrusion zone. Inasmuch as similar units of the Acatlán Complex bound this HP root zone on either side, it is inferred to have been extruded into the upper plate above the subduction zone, and thus is not an oceanic suture. Our new data provide constraints for a Carboniferous palaeogeographic reconstruction, whereby subduction erosion of passive margin rocks took place along the western margin of Pangea and were subsequently extruded into the upper (Acatlán) plate.

Exhumation of the Diancang Shan metamorphic complex along the Ailao Shan-Red River belt, southwestern Yunnan, China: Evidence from 40Ar/ 39Ar thermochronology

Exhumation of the high-grade metamorphic rocks from deep to shallow crustal levels in the Diancang Shan (DCS) massif is evident by the temporal transition from shearing and mylonitization at the amphibolite facies in the lower to middle crust, through retrograde ductile–brittle faulting at greenschist facies in the middle crust and brittle faulting near the surface. New 40Ar/ 39Ar ages of amphibole, muscovite, biotite and K-feldspar, mostly from mylonitic rocks, help constrain the thermal evolution of the massif during its exhumation. The thermochronologic studies have shown at least three stages of exhumation and cooling from late Oligocene to Pliocene, e.g., 28–13 Ma, 13–4 Ma and 4–0 Ma, respectively. The initiation of the unroofing history of the DCS massif resulted from ductile left-lateral shearing along the Ailao Shan-Red River shear zone (ASRR) since the late Oligocene. Diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history of the massif. The first stage of exhumation might have stopped due to the cessation of the left-lateral shearing at ca. 21 Ma, although cooling continued until 13 Ma ago to relatively low-temperature conditions. The second and third stages of cooling started from 13 Ma and lasted until the recent active faulting. The diversity of cooling rates during the second stage from 13 to 4 Ma suggests differences in exhumation rates at various localities. Ductile to brittle shearing along the eastern flank retrograde normal shear zone is the best explanation for the exhumation during this stage. The inhomogeneous cooling and exhumation of the lower plate of the normal fault zone possibly resulted from the tilting of the lower plate or differential exhumation of slabs in the lower plate. During the third stage, cooling and exhumation of the DCS massif since 4 Ma ago is recognized when higher temperature cooling paths are extended and plotted at a lower temperature of about 16 °C, the present day average surface temperature. Therefore, the diachronous cooling and exhumation continuously spanned to the surface. The tectonic exhumation of the DCS massif constitutes a part of the tectono-thermal evolution and diachronous exhumation of the metamorphic complexes in the fan-shaped area bounded broadly in SE Asia by the Red River fault zone and Sagaing fault zone since 36 Ma. Extrusion-induced strike-slip shearing and rotation-associated normal faulting, played important roles in the diachronous cooling and exhumation of the DCS massif and other metamorphic complexes in SE Asia.

Geochronologic evidence for a cold arc–continent collision: The Taiwan orogeny

In order to constrain the tectonic consequences of the collision of the Luzon arc with the SE Chinese continental margin during the Neogene Taiwan orogeny, we assess the thermal history of the poly-metamorphic para-autochtthonous rocks of the continental margin in Taiwan. To accomplish this we report a new SIMS U–Pb age of prograde metamorphic zircons and interpret this age in the context of existing geochronology and thermochronology. Zircons extracted from the Fenniaolin amphibolite near Nanao, Taiwan, in the contact aureole of a granodiorite, are interpreted to have crystallized during the first lower amphibolite facies prograde metamorphism based on [1] their equant shape rather than the prismatic shape typical of magmatic zircon, [2] their microcrystalline or “micro-saccharoidal” aggregate structure and patchy and sector zoning rather than oscillatory zoning visible in cathodoluminescent imaging, [3] their low concentrations of U (most<40ppm), [4] their chlorite inclusions, and [5] their inclusion within prograde metamorphic amphiboles. The apparent absence of magmatic zircon is also consistent with the low Zr concentrations of the N-MORB metabasaltic protolith. SIMS analyses of 18 zircon grains yield a concordant 206Pb/238U age of 86±1Ma, analytically indistinguishable from the age of the adjacent pluton that caused the contact metamorphism. We thus conclude that the contact metamorphism of this intrusion caused the first lower amphibolite facies metamorphism of these rocks in the mid-Cretaceous.Existing 40Ar/39Ar and Rb–Sr cooling ages of both amphibole and muscovite at ~80Ma (e.g. Lan, 1990; Lo and Onstott, 1995) show that the granodiorite and its contact aureole cooled rapidly from ~700° to <400°C in~5m.y., indicating that the ambient temperature of the country rocks at the time of intrusion was less than the closure temperature of Ar diffusion in muscovite. Existing thermobarometry on rocks in the contact aureole yield pressures of ~5kb, equivalent to a depth of 15–20km. Cooling ages of biotite and K-feldspar in the literature reflect slow cooling at <2°C/m.y. for more than 50m.y. to at least 20Ma, suggesting atectonic cooling.The crustal history reconstructed from these data shows that the regional ‘gneissic’ terrane of the northern Tananao complex may have been caused by coalescing contact aureoles of multiple mid-Cretaceous intrusions (the ‘Nanao intrusive event’), and not by the Neogene Taiwan orogeny. On the contrary, the Tananao rocks probably achieved middle greenschist conditions and 15–20km depth in a forearc setting in the Early Cretaceous. This intrusive event caused ‘regional contact metamorphism’ at about 85Ma, after which the passive margin setting preserved these rocks at mid-crustal depths until the beginning of the Taiwan orogeny (~5Ma). These depths (10–12km) and temperatures (~200–250°C) were high enough to account for the late Miocene lower greenschist facies cleavages and the Pliocene–Pleistocene fission track cooling ages without additional loading and heating from the Taiwan orogeny.

Zirconology of nepheline syenite of the Berdyaush Massif (Southern Urals)

Problems of the origin of rocks of the BerdyaushMassif including nepheline syenite (NS) have beenconsidered many times [1–5], which allows us torestrict ourselves to a short description of only thosedata directly related to the mentioned problem. A.N.Zavaritskii [1] distinguished two stages in the formation of the massif: the earlier stage including intrusionsof basic and granite magmas, the formation of hybridrocks and rapakivi, and the later stage comprisingintrusions of syenite and related NS. In [2] thissequence is complemented by the appearance ofalkalirich basalt, as well as the formation of dykes ofdiabasic porphyrite, orthophyre, bostonite, and NS inthe Late Riphean–Early Vendian. In essence, theseslightly different models illustrate the controversialnature of the problem of NS origin and their relationswith other rocks of the massif.Geochronological information about NS does notgive a direct answer to these questions. The K–Ar ageof amphibole from NS (1370 Ma) is considered asconfirmation of their relationships with rapakivi, anddating of 670–716 Ma is related to the processes ofalbitization and recrystallization of NS [3]. Later [4]not only new K–Ar data (1245–1347 Ma), but Rb–Srdata of 1350 ± 17 Ma (formation) and 750 Ma (metasomatic transformations) as well were obtained withNS. The first data on zircons from NS (Sample K633,Golaya Mt.) appeared in 1986 [5]. Their mineralogicalproperties were described; crushing, replacement ofearly generations by late ones with synchronousdecrease of U concentrations from 2890 to 970 ppm inthem, and geochemical heterogeneity in the distribution of Th, Y, and Fe were registered. Attention waspaid to the violation of isotopic age relationships inzircons and clearly reflected differences of theirappearance, composition, and sizes from such zirconsof other rocks of the massif significantly depleted in U(96–207 ppm). The classic U–Pb method by zirconsprovided the age parameters of the NS formation (1354 ±20 Ma) and their metamorphism (750–800 Ma).The development of new analytical methods madeit possible to date not only single crystals, but theirindividual parts as well, which significantly widenedthe possibilities to obtain age information on heterogeneous crystals. Using the possibilities of a SHRIMPmicroprobe, the age of zircons from rocks related tonepheline syenite from the Berdyaush Massif wasdetermined as 1373 ± 21 Ma [6]. Although it was closeto the previously obtained ones, the age problem wasnot solved. Besides that, the absence of the information about the composition and location of the studiedrocks in [6] did not allow us to carry out the comparative analysis of the studied rocks and estimate theirconsistence to each other. Clearly it was necessary toconfirm the age information for zircons (Sample K633) previously obtained by the classic U–Pb methodby datings of individual crystals. Using the knownSHRIMP methodology [7, 8], the results obtained forthis sample are given in table and in Figs. 1–3. It is evident that they are consistent with the previouslyobtained results and in addition more clearly reflectthe development of the secondary porous substance atthe expense of the primary homogeneous one. Isotopic analyses of both phases were performed for Crystal 5;only U and Th were measured for crystals 9 and 10. Thetotal crystal community was divided into three groups(Fig. 2): (I) nebular, porous (3, 5.1, 6, 8, and 9);(II) intermediate (1 and 5.2); (III) homogeneous,transparent (2, 4, 7, and 10). Zircons I were significantly depleted in U and Th (U

The multistage exhumation history of the Kaghan Valley UHP series, NW Himalaya, Pakistan from U-Pb and 40Ar/39Ar ages

Amphibole and mica 40Ar/39Ar ages as well as zircon, rutile and titanite U-Pb geochronology of eclogites and associated host rocks from the Higher Himalayan Crystalline Nappes (Indian Plate) in the Upper Kaghan Valley, Pakistan allow distinction of a multistage exhumation history. An Eocene age for peak-pressure metamorphism has been obtained by phengite 40Ar/39Ar (47.3 ± 0.3 Ma) and zircon U-Pb (47.3 ± 0.4 and 47.4 ± 0.3 Ma) ages from cover and basement gneisses. A very short-lived metamorphic peak and rapid cooling is documented by an amphibole 40Ar/39Ar age of 46.6 ± 0.5 Ma and a rutile U-Pb age of 44.1 ± 1.3 Ma from eclogites. Phengite and biotite ages from cover and basement sequences metamorphosed during the Himalayan orogeny are 34.5 ± 0.2 to 28.1 ± 0.2 Ma whereas youngest biotites, yielding 23.6 ± 0.1 and 21.7 ± 0.2 Ma, probably reflect argon partial resetting. The amphibole age, together with those derived from phengite and zircon demonstrate a rate of initial exhumation of 86–143 mm/a i.e . an extremely rapid transport of the Indian Plate continental crust from ultra-high pressure (UH P ) conditions back to crustal levels (47–46 Ma for transport from 140 to 40 km depth). Subsequent exhumation (46–41 Ma, 40–35 km) slowed to about 1 mm/a at the base of the continental crust but increased again later towards slightly higher exhumation rates of ca . 2 mm/a (41–34 Ma, 35–20 km). This indicates a change from buoyancy-driven exhumation at mantle depths to compression forces related to continent-continent collision and accompanied crustal folding, thrusting and stacking that finally exposed the former deeply-buried rocks.

Quartz-albite-microcline metasomatic rocks in the Main Sayan Fault Zone: Evolution of metasomatism and composition of accessory minerals

Quartz-albite-microcline metasomatic rocks (qualmites) localized in the Main Sayan Fault Zone at the boundary of Irkutsk oblast and Buryatia (Eastern Sayan, middle reaches of the Kitoi River) were formed after dynamometamorphic biotite and biotite-amphibole granite gneisses. The zone of alkaline metasomatism up to 250–300 m thick extends for about 12 km along the Main Sayan Fault. Riebeckite-aegirine and hornblende-clinopyroxene qualmites were formed during the early alkaline stage of metasomatism. Biotite-magnetite qualmites, which are occasionally superimposed on riebeckite-aegirine rocks, are products of the following stage of increasing acidity. The 40Ar/39Ar age of amphibole from metasomatic rocks is 321 ± 5 Ma. The metasomatic rocks are enriched by 2–4 times in Zr, Nb, Y, REE, Be, Th, and U relative to unaltered granite gneisses and contain rare-metal mineralization (fergusonite-(Y), betafite, Nb- and Y- bearing titanites, gadolinite-(Y), zircon, thorite, allanite-(Ce), chevkinite-(Ce), etc.). The composition of accessory minerals was studied on a LEO-1430VP SEM. Their composition and mineral assemblages show that meta-somatic alteration and Ti-Nb-Zr-REE mineralization were formed synchronously. The stage of acid solution neutralization was characterized by crystallization of epidote and andradite and by replacement of chevkinite with allanite and titanite in metasomatic rocks. Hydro- and fluorcarbonates of LREE, phosphates (monazite-(Ce)), and fluorides (F-bearing thorianite?) were formed during the final low-temperature stage.

The Yermakovsky beryllium deposit, Western Transbaikal region, Russia: Geochronology of igneous rocks

The sequence of rock and ore formation at the Yermakovsky beryllium deposit is established on the basis of geological relationships and Rb-Sr and U-Pb isotopic dating. The Rb-Sr age of amphibolitefacies regional metamorphism is determined for quartz-biotite-plagioclase schist (266 ± 18 Ma) and dolomitized limestone (271 ± 12 Ma) of the Zun-Morino Formation. The U-Pb zircon age of premineral gabbro is 332 ± 1 Ma. The Rb-Sr age of gabbro is somewhat younger (316 ± 8.3 Ma), probably owing to the effect of Hercynian metamorphism on sedimentary rocks of the Zun-Morino Formation and gabbroic intrusion that cuts through it. The U-Pb zircon age of gneissose granite of the Tsagan Complex at the Yermakovsky deposit is 316 ± 2 Ma, i.e., close to the age of metamorphism superimposed on gabbro rocks. The U-Pb zircon age of preore granitic dikes, estimated at 325 ± 3 and 333 ± 10 Ma, is close to the age of gabbro. The Ar/Ar age of amphibole from a granitic dike (302.5 ± 0.9 Ma) probably displays a later closure of this isotopic system or the effect of superimposed processes. The Rb-Sr age of alkali syenite intrusion is 227 ± 1.9 Ma. The U-Pb zircon age of alkali leucogranite stock pertaining to the Lesser Kunalei Complex is 226 ± 1 Ma, while the Rb-Sr age of beryllium ore is 225.9 ± 1.2 Ma. These data indicate that beryllium ore mineralization is closely related in space and time to igneous rocks of the Lesser Kunalei Complex dated at 224 ± 5 Ma and varying from gabbro to alkali granite in composition. Thus, the preore Hercynian magmatism at the Yermakovsky deposit took place ∼330 Ma ago and was completed by metamorphism dated at 271–266 Ma. The ore-forming magmatism and beryllium ore mineralization are dated at 224 ± 5 Ma. Postore magmatic activity is scarce and probably correlated with tectonic melange of host rocks.

Emplacement age of the Songshugou ultramafic massif in the Qinling orogenic belt, and geologic implications

The Songshugou ultramafic massif is located to the north of the Shang‐Dan fault, the Palaeozoic suture between the North and South China blocks. It is the largest Apline‐type ultramafic body in the Qinling orogenic belt of central China, consisting mainly of dunite with a small amount of harzburgite and minor pyroxenite. We present new LA‐ICP‐MS U−Pb dating and trace element results for zircon from two garnet amphibolite samples in the contact metamorphic zone surrounding the massif. One was sampled ∼1 m from the massif, the other ∼5 m away. The studied zircon grains are small, anhedral, and display typical metamorphic characteristics of low Th/U values (<0.1). The U and Th concentrations of zircon range from several hundred ppm to less than 10 ppm. Cathodoluminescence images show two apparent generations of zircon, with lighter cores and darker rims. Core and rim ages however, are identical within error. These two samples yield identical concordant ages of 506±7 and 510±7 Ma, suggesting that the Songshugou ultramafic massif was emplaced at ∼510 Ma. Low HREE concentrations and the absence of Eu anomalies in most analysed zircons suggest that the studied grains most likely formed during garnet amphibolite metamorphism induced by emplacement of the ultramafic massif. To better understand the cooling history of the massif, 40Ar/39Ar ages of amphibole from three garnet amphibolite specimens in the contact metamorphic zone and one amphobolite sample about 20 m away from the massif were determined. The 40Ar/39Ar ages increase from 372±15 Ma (JSM‐01) near the massif to disturbed, unreliable ‘plateau’ ages of 474±8 Ma (JSM‐03) and 781±146 Ma (JSM‐04) with increasing distance from the ultramafic massif, showing limited heating during exhumation of the massif, followed by slow cooling. Therefore, the Songshugou ultramafic massif does not reflect the Jining orogeny at ∼1 Ga. Instead, it was emplaced into the Proterozoic, Qinling Group during the Palaeozoic, probably due to the subduction along the Shang‐Dan fault.

40Ar– 39Ar geochronology across Archean and Paleoproterozoic terranes from southeastern Guiana Shield (north of Amazonian Craton, Brazil): Evidence for contrasting cooling histories

A 40Ar/ 39Ar geochronological study was performed on amphibole and biotite from some representative units of distinct tectonic domains of the southeastern Guiana Shield, north of the Amazonian Craton, the Amapá Block and the Carecuru Domain. In the Amapá Block, an Archean continental block involved in the Transamazonian orogenesis (2.26–1.95 Ga), the investigated minerals, from rocks of the Archean high-grade basement assemblage, give only Paleoproterozoic ages, indicating their complete resetting during the Transamazonian orogenic event. Amphibole ages vary from 2087 ± 3 to 2047 ± 20 Ma, and biotite ages spread mainly between 2079 ± 18 and 2033 ± 13 Ma. In the Carecuru Domain, in which the geodynamic evolution is related to Paleoproterozoic magmatic arc setting during the Transamazonian event, calc-alkaline granitoids yield amphibole age of 2074 ± 17 Ma, and biotite ages of 1928 ± 19 Ma and 1833 ± 13 Ma. These data reinforce the importance of the Transamazonian orogenic cycle in the investigated area, and indicate that the rocks were not significantly affected by post-Transamazonian events. When coupled with available U–Th–Pb monazite and Pb–Pb zircon geochronological records and petro-structural observations, the new 40Ar/ 39Ar data delineate contrasting cooling and exhumation histories for the tectonic domains. In the Amapá Block, the data suggest nearly vertical T– t paths that reflect fast cooling rates, which indicate tectonically controlled exhumation, related to collisional stages of the Transamazonian event, between 2.10 and 2.08 Ga. Conversely, in the Carecuru Domain, low cooling rates suggest that the arc-related granitoids underwent slow and monotonous cooling since their emplacement until reaching the biotite isotopic closure temperature.

Permian 40Ar/39Ar ages for post-Variscan minor intrusions in the Iberian Range and Spanish Central System

40Ar/39Ar dating of biotite from a calc-alkaline gabbro stock NW of Loscos in the Iberian Range yielded a slightly disturbed gas release spectrum. It has an early Permian, 288.5 ± 1.4 Ma total gas age, interpreted as the minimum emplacement age. The 40Ar/39Ar apparent age of the gabbro agrees with the Autunian stratigraphic age of related volcanic rocks. 40Ar/39Ar step-heating dating of amphibole phenocrysts from an alkaline camptonite dyke ca. 4 km SE of El Hoyo de Pinares in the Sierra Guadarrama sector of the Spanish Central System yielded a mid-Permian 264 ± 3 Ma plateau age, interpreted as the emplacement age. The age is indistinguishable from published 40Ar/39Ar amphibole ages for N-S trending camptonite dykes from the western, Sierra de Gredos segment of the Central System. This suggests that the camptonite dykes from both sectors belong to the same generation.