Inherited Zircon Cores Research Articles

Garnet Lu[sbnd]Hf geochronology and P-T path of the Gridino-type eclogite in the Belomorian Province, Russia

Eclogites are commonly seen as markers of subduction and thus their presence in Proterozoic and Archean orogenic provinces is crucial information for determining the initiation of modern plate tectonic. The Belomorian province hosts some of the oldest known eclogite-facies rocks. Here we present new garnet LuHf geochronology that constrains the prograde stage of Gridino-type eclogite to ca. 1.96–1.92 Ga. Inherited magmatic zircon cores have 176Hf/177Hf ratios that indicate isotopic disequilibrium between the inherited zircon cores and the metamorphic mineral assemblages, while the metamorphic rims have variable 176Hf/177Hf ratios that reflect Paleoproterozoic eclogite-facies metamorphism. Iterative thermodynamic models, involving the optimization of pressure (P), temperature (T) and reactive bulk composition (X) was used to reconstruct the P-T conditions recorded by garnet, and define the prograde trajectory. Zr-in-rutile thermometry, combined with equilibrium phase diagrams, constrains the peak P-T conditions at 725–750 °C and above 18 kbar, corresponding to an average apparent thermal gradient of below 41 °C/kbar. The P-T conditions of the high-pressure granulite facies overprint are 680–730 °C at 9–10 kbar. Thus, the metamorphic evolution of Gridino-type eclogite followed a clockwise P-T path with a cooling decompression stage, recording the assembly of the Columbia supercontinent during the Paleoproterozoic.

From Jurassic rifting to Cretaceous subduction in NW Iranian Azerbaijan: geochronological and geochemical signals from granitoids

Previous interpretations of a Jurassic subduction in Iran were based on trace element classification diagrams for granitoids, but their reliability is questionable, underscored by modern examples of continental break-up zones such as the Baja California. We present new field observations, bulk rock geochemistry, Sr and Nd isotope analyses and U–Pb zircon geochronology to assess the age and tectonic setting of previously undated intermediate to felsic magmatic rocks cropping out in the Precambrian basement of NW Iranian Azerbaijan. The geochronology revealed an uneven distribution in space and time: Late Jurassic (159–154 Ma) intrusions and dikes are alkaline to calc-alkaline. Their melt source is mantle dominated with a distinct continental contribution disclosed by radiogenic isotopes and abundant inherited zircon cores. Mid-Cretaceous (112–96 Ma) plutonic bodies and associated volcanic rocks occur only to the east of the major Siah Cheshmeh–Khoy Fault. They have geochemical signatures typical of a metasomatized mantle. In consistence with the sedimentation history of the area, our new interpretation attributes the Late Jurassic magmatism to thinning of a continental lithosphere in a rift-related setting. Mid-Cretaceous magmatism was produced by oceanic subduction beneath the Central Iran continent. We interpret the 40-Ma age gap between the two magmatic episodes as the time of opening of the oceanic basin witnessed by the Khoy ophiolite in the study area.

Zircon Hf-O isotopic constraints on the origin of Late Mesozoic felsic volcanic rocks from the Great Xing'an Range, NE China

The voluminous Late Mesozoic magmatism was related to extensive re-melting of juvenile materials that were added to the Central East Asia continent in Phanerozoic time. The most favoured magma generation mechanism of Late Mesozoic magmas is partial melting of underplated lower crust that had radiogenic Hf-Nd isotopic characteristics, but this mechanism faces difficulties when interpreting other isotopic data. The tectonic environment controlling the generation of the Late Mesozoic felsic magmas is also in dispute. In this study, we obtained new U-Pb ages, and geochemical and isotopic data of representative Jurassic (154.4 ± 1.5 Ma) and Cretaceous (140.2 ± 1.5 Ma) felsic volcanic samples. The Jurassic sample has inherited zircon cores of Permian age, with depleted mantle-like εHf(t) of +7.4 − +8.5, which is in contrast with those of the magmatic zircons (εHf(t) = +2.4 ± 0.7). Whereas the inherited cores and the magmatic zircons have identical mantle-like δ18O composition ranges (4.25–5.29‰ and 4.69–5.54‰, respectively). These Hf-O isotopic characteristics suggest a mixed source of enriched mantle materials rather than ancient crustal components and a depleted mantle source represented by the inherited Permian zircon core. This mechanism is manifested by the eruption of Jurassic alkaline basalts originated from an enriched mantle source. The Cretaceous sample has high εHf(t) of +7.0 − +10.5, suggesting re-melting of a mafic magma derived from a depleted mantle-source. However, the sub-mantle zircon δ18O values (3.70–4.58‰) suggest the depleted mantle-derived mafic source rocks had experienced high temperature hydrothermal alteration at upper crustal level. Therefore, the Cretaceous felsic magma, if not all, could be generated by re-melting of down-dropped supracrustal volcanic rocks that experienced high temperature oxygen isotope alteration. The two processes, enriched mantle-contribution and supracrustal juvenile material re-melting, are new generation mechanisms of the Late Mesozoic magmas from Central East Asia. Rift settings may have controlled these processes throughout crustal and mantle levels.

The Paleoproterozoic Vishnu basin in southwestern Laurentia: Implications for supercontinent reconstructions, crustal growth, and the origin of the Mojave crustal province

The formation of the Mojave crustal province has been a persistent enigma in models of the Proterozoic tectonic history of southwestern Laurentia. It is composed of similarly-aged 1.8–1.7 Ga rocks as the adjacent Yavapai province, yet shows evidence of much older (>2.2 Ga) lithospheric components in all isotopic systems. We present >700 new U-Pb analyses and >350 new Lu-Hf analyses of zircon from the oldest metasedimentary and plutonic rocks of the Mojave province to better understand its origin and evolution. Six metasedimentary rocks have detrital zircon age populations dominated by ∼1.87 and 2.4–2.7 Ga grains. This age distribution is like the Vishnu Schist in Grand Canyon and we suggest that together they comprise a regional turbidite basin that we name the Vishnu basin. Cross cutting relationships indicate that deposition of the Vishnu basin proceeded from west to east (present coordinates) from ∼1.8 to 1.75 Ga. The Vishnu basin extends from central Arizona to the Transverse Ranges in California and possibly beyond the present boundaries of Laurentia to previously adjacent cratonic blocks. We propose the Mawson continent as the source of Vishnu basin detritus, and hence favor Nuna accretion at ∼1.8 Ga. Paleoproterozoic plutons that intrude the Vishnu basin sampled in this study range in age from 1791 ± 15 to 1691 ± 15 Ma. Plutons show a systematic change in Hf-isotope composition through time and space. The oldest plutons in the western Mojave province have the most isotopically evolved signatures and contain inherited zircon cores and xenocrysts with age and Hf-isotope characteristics that suggest they were derived from Vishnu basin sediments and/or 1.8–2.7 Ga lower crust. The Hf-isotope composition of plutons becomes more radiogenic (juvenile) from west-to-east and from old-to-young. The magnitude of Hf-isotope variation requires increased influence of depleted mantle sources through time. Hf-isotope compositions of the younger 1.7–1.68 Ga syn-to-post orogenic granites show more evolved compositions attributed to lower crustal melting due to crustal thickening during the Yavapai orogeny.

Metasomatized mantle as the source of Mid-Miocene-Quaternary volcanism in NW-Iranian Azerbaijan: Geochronological and geochemical evidence

Middle Miocene to Quaternary volcanic rocks cover large areas of the Azerbaijan Province in NW Iran. This study reports two separate age clusters out of 23 new LA-ICP-MS U-Pb zircon ages: (1) Middle Miocene (16.2–10.6Ma) and (2) Latest Miocene-Late Pleistocene (5.5–0.4Ma). Major and trace element bulk rock geochemistry and initial Sr, Nd, Pb radiogenic isotope data on the dated rocks provide new constraints on the Mid-Miocene to Quaternary volcanism in this region. The analyses are distributed over a large compositional range from low-K to high-K calc-alkaline andesites and dacites/rhyolites to more alkaline trachybasalts and dacites with shoshonitic affinities. Chondrite-normalized REE patterns are steep with significant enrichment in LREE and low abundances of HREE indicating a garnet control. Plots of primitive mantle-normalized trace elements show negative Ti and Nb-Ta anomalies indicative of an arc signature. The wide compositional range and the ubiquitous presence of an arc signature reveal that the source mantle is heterogeneous and metasomatically altered. Sr, Nd and Pb radiogenic isotope data further point towards an enriched mantle source and/or crustal contamination. Crustal contamination is best recognized by inherited zircon cores, which yield Late Neoproterozoic to Early Cambrian ages typical for the Iranian basement. The occurrence of adakite-like compositions with elevated magnesium numbers, Cr and Ni concentrations argue against a fractionation-driven process but point to a subcrustal origin. Overall, the analyzed lavas show no spatial and temporal relation to a potential subduction zone, confirming the dated volcanics to be post-collisional and not related to singular processes such as slab retreat or delamination of a continuous lower crustal sliver. We propose three hypotheses to explain the reported disparity in distribution, age and composition and favour small-scale sublithospheric convection or incorporation of crustal material into the metasomatized mantle. The discovery of the late Miocene time gap is in line with previously advocated exhumation pulses and coincides with a major tectonic reorganization in the Arabian-Eurasian realm at this time.

The late-Variscan peraluminous Valdepeñas pluton (southern Central Iberian Zone)

The Valdepenas pluton is the easternmost outcrop of the Caceres-Valdepenas magmatic alignment (southern Central Iberian Zone). This massif is constituted by a cordierite-bearing porphyritic monzogranite and may be grouped within the so-called “Serie Mixta” granitoids. The Valdepenas monzogranite is of magnesian [FeO t /(FeO t +MgO)~0.76], alkali-calcic [(Na 2 O+K 2 O)–CaO=7.8–8.5] and peraluminous (A/CNK=1.14–1.20). Multielemental- and REE-normalized patterns are comparable to those of similar rocks in the Nisa- Alburquerque-Los Pedroches magmatic alignment, and slightly differ from those of the Montes de Toledo batholith, both in the southern Central Iberian Zone. The U-Pb zircon age of 303±3Ma is consistent with the late-orogenic character of the intrusion and is in accordance with most of the granitic peraluminous intrusions in the southern Central Iberian Zone. 86 Sr/ 87 Sr 300Ma ratios (0.707424–0.711253), eNd 300Ma values (-5.53 to -6.68) and whole-rock major and trace element compositions of the studied rocks, suggest that the parental magma of the Valdepenas monzogranite could derive from a crustal metaigneous source. The U-Pb ages (552–650Ma) of inherited zircon cores found in Valdepenas monzogranite samples match those often found in Lower Paleozoic metavolcanics and granitic orthogneisses of Central Iberia and, furthermore, point to Upper Neoproterozoic metaigneous basement rocks as possible protoliths at the magma source. Based on the solubility of monazite in peraluminous melts, the estimated emplacement temperature of the studied monzogranite is 742–762oC. The results obtained in this work would contribute to a better understanding of the origin of the “Serie Mixta” granitoids.

Eocene magmatism from the Liemai intrusion in the Eastern Tethyan Himalayan Belt and tectonic implications

AbstractMultistage magmatic thermal events occurred in the Yardoi Dome and contain important information on the tectonomagmatic processes. The dome has played a crucial role in understanding the collisional evolution of the Tethyan Himalayan. We present new geochronological and geochemical data for muscovite-granite exposed in the Liemai area, Eastern Tethyan Himalayan Belt. Liemai muscovite-granite is strongly peraluminous, with A/CNK values characterized by evolved geochemical composition with high contents of SiO2-enriched large-ion lithophile elements, and is depleted of high-field-strength elements. These geochemical features indicate that granites possibly derived from partial melting of metasedimentary rocks and plagioclase fractional crystallization probably played a critical role in production of peraluminous granitic melts. Zircon U–Pb dating from muscovite-granite yielded ages of approximately 48.5 ± 1.1 Ma, representing its crystallization ages. This age is the oldest age of Tethyan Himalayan leucogranite from the Yardoi Dome and adjacent areas. However, the inherited zircon cores have ages of 135.7–3339.2 Ma. The εHf(t) values of all zircons vary from –6.4 to –2.3 and have varying Hf-isotope crustal model ages of 731–839 Ma. The geochemical and isotopic compositions indicate that magma of the Liemai granite can most likely be interpreted as products of the break-off related to thermal perturbation along the break-off window associated with the subduction of Neo-Tethyan slab. These magmas were derived mainly from the anatexis of ancient crustal materials under contraction and thickening conditions due to subduction of the Indian continent beneath southeastern Tibet.

Origin of the Bashierxi monzogranite, Qiman Tagh, East Kunlun Orogen, NW China: A magmatic response to the evolution of the Proto-Tethys Ocean

The Qiman Tagh of the East Kunlun Orogen, NW China, lies within the Tethysides and hosts a large W–Sn belt associated with the Bashierxi monzogranite. To constrain the origin of the granitic magmatism and its relationship with W–Sn mineralization and the tectonic evolution of the East Kunlun Orogen and the Tethys, we present zircon U–Pb ages and Hf isotopic data, and whole-rock compositional and Sr–Nd–Pb isotopic data of the Bashierxi monzogranite. The granite comprises quartz, K-feldspar, plagioclase, and minor muscovite, tourmaline, biotite, and garnet. It contains high concentrations of SiO2, K2O, and Al2O3, and low concentrations of TiO2 and MgO, indicating a peraluminous high-K calc-alkaline affinity. The rocks are enriched in Rb, U, Pb, and light rare earth elements, and relatively depleted in Eu, Ba, Nb, Sr, P, and Ti, and are classified as S-type granites. Twenty zircon grains yield a weighted mean 238U/206Pb age of 432±2.6Ma (mean square weighted deviation=1.3), indicating the occurrence of a middle Silurian magmatic event in the region. Magmatic zircons yield εHf(t) values of −6.7 to 0.7 and corresponding two-stage Hf model ages of 1663–1250Ma, suggesting that the granite was derived from Mesoproterozoic crust, as also indicated by 207Pb/206Pb ages of 1621–1609Ma obtained from inherited zircon cores. The inherited zircon cores yield εHf(t) values of 8.3–9.6, which indicate the generation of juvenile crust in the late Paleoproterozoic. Samples of the Bashierxi granite yield high initial 87Sr/86Sr ratios and radiogenic Pb concentrations, and negative εNd(t) values. Isotopic data from the Bashierxi granite indicate that it was derived from partial melting of ancient (early Paleozoic to Mesoproterozoic) sediments, possibly representing recycled Proterozoic juvenile crust. Middle Silurian granitic magmatism resulted from continental collision following closure of the Proto-Tethys Ocean. The Qiman Tagh represents a Caledonian orogenic belt containing S-type granites and associated W–Sn deposits.

The Serra das Araras Batholith: An example of Ediacaran syntectonic peraluminous granitic magmatism in the southwestern Paraíba do Sul Domain

The Serra das Araras Batholith (SAB), located at the southwest portion of the Paraíba do Sul Domain, Central Ribeira Belt, is composed of orthogneisses with peraluminous character. Combined LA-ICP-MS U-Pb and Lu-Hf isotopic analyses have been performed in zircon grains for a representative sample (SA-3H) of SAB. Data point to Rhyacian to Orosirian (ca. 2.1–1.8 Ga) inherited zircon cores with positive ɛHf(t) values and Ediacaran U-Pb crystallization age (595 ± 8 Ma). A leucogranite dyke (SA-3D), 15–25 cm thick, which crosscuts SAB and a biotite-hornblende quartz diorite enclave (SA-06) have been analyzed. Crystallization ages for the three samples (ca. 600-590 Ma) and ɛHf(t) values, ranging from −5.6 to −29.4 for Neoproterozoic zircon grains, indicate an Ediacaran crustal magmatism involving mid to lower crust anatexis and the reworking of Paleoproterozoic juvenile and crustal rocks.

S-type granite generation and emplacement during a regional switch from extensional to contractional deformation (Central Iberian Zone, Iberian autochthonous domain, Variscan Orogeny)

Zircon grains extracted from S-type granites of the Meda-Escalhao-Penedono Massif (Central Iberian Zone, Variscan Orogen) constrain the timing of emplacement and provide information about potential magma sources. Simple and composite zircon grains from three samples of S-type granite were analyzed by LA-ICP-MS. New U–Pb data indicate that granites crystallized in the Bashkirian (318.7 ± 4.8 Ma) overlapping the proposed age range of ca. 321–317 Ma of the nearby S-type granitic rocks of the Carrazeda de Anciaes, Lamego and Ucanha-Vilar massifs. The timing of emplacement of such S-type granites seems to coincide with the waning stages of activity of a D2 extensional shear zone (i.e. Pinhel shear zone) developed in metamorphic conditions that reached partial melting and anatexis (ca. 321–317 Ma). Dykes of two-mica granites (resembling diatexite migmatite) are concordant and discordant to the compositional layering and S2 (main) foliation of the high-grade metamorphic rocks of the Pinhel shear zone. Much of the planar fabric in these dykes was formed during magmatic crystallization and subsequent solid-state deformation. Field relationships suggest contemporaneity between the ca. 319–317 Ma old magmatism of the study area and the switch from late D2 extensional deformation to early D3 contractional deformation. Inherited zircon cores are well preserved in these late D2-early D3 S-type granite plutons. U–Pb ages of inherited zircon cores range from ca. 2576 to ca. 421 Ma. The spectra of inherited cores overlap closely the range of detrital and magmatic zircon grains displayed by the Ediacaran to Silurian metasedimentary and metaigneous rocks of the Iberian autochthonous and parautochthonous domains. This is evidence of a genetic relationship between S-type granites and the host metamorphic rocks. There is no substantial evidence for the addition of mantle-derived material in the genesis of these late D2–early D3 S-type granitic rocks. The eNd arrays of heterogeneous crustal anatectic melts may be just inherited from the source, probably reflecting mixing of a range of crustal materials with different ages and primary isotopic signatures. The generation of the Bashkirian S-type granites has been dominated by continental crust recycling, rather than the addition of new material from mantle sources.

Late Mesozoic high-K calc-alkaline magmatism in Southeast China: the Tongling example

ABSTRACTWe report new zircon U–Pb ages, Hf isotopic and geochemical results for the Tongling granitic plutons of Southeast China. SHRIMP U–Pb ages for the Miaojia quartz monzodiorite porphyrite，the Tianebaodan and Tongguanshan quartz monzodiorites, the Xinqiaotou granodiorite porphyry, and the Shatanjiao and Nanhongchong granodiorite are 143 ± 2, 141 ± 1 and 142 ± 1, 147 ± 1, and 145 ± 1 and 139 ± 1 Ma, respectively. Combined with previous geochronological data, our results indicate that the porphyritic rocks are older than rocks of the same type lacking porphyritic texture. Geochemically, these high-K calc-alkaline intrusive rocks are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Lu–Hf isotopic compositions of zircons from the high-K calc-alkaline (HKCA) rocks have εHf(t) values of magmatic 139–147 Ma zircons from −8.1 to −25.6, with two-stage model ages (tDM2) of 1.71–2.67 Ga, whereas εHf(t) values of inherited 582–844 Ma zircons range from 5.4 to −9.5, with tDM2 of 1.39–2.22 Ma, younger than tDM2 values of igneous zircon, indicating that newly formed mantle material was added to the continental crust of the Yangtze Block. Moreover, εHf(t) values of inherited zircon cores older than 1000 Ma are from −7.8 to −26, similar to magmatic zircons, and the tDM2 values are all greater than 3.0 Ga (3.16–3.75 Ga), reflecting partial melting of ancient sialic material. We conclude that the plutonic melts were derived from both the enriched mantle and the ancient crust. The HKCA Tongling intrusions coincide temporally with the J3–K1 magmatic event that was widespread in Southeast China. This igneous activity may have accompanied sinistral slip along the Tan-Lu fault due to oblique subduction of the Palaeo-Pacific plate.

Lu-Hf isotope composition of zircon as an indicator of the sources for Paleoproterozoic collisional granites (Sharyzhalgai uplift, Siberian craton)

We present geochemical characteristics of rocks and results of local dating and Lu-Hf isotopic analysis of zircons from two plutons of Paleoproterozoic collisional granitoids in the northwest of the Sharyzhalgai uplift. The rocks of the Alar pluton in the Bulun terrane correspond in major- and trace-element composition to I-type potassic granites. The Alar granites formed at ~780 °C and <5-8 kbar through melting of predominantly graywacke (volcanosedimentary) source rocks with the contribution of melt from plagiogneisses of tonalite-trondhjemite complex. The age and Lu-Hf isotopic similarity between inherited zircon core (3.3-3.0 and 2.85-2.6 Ga) in these granites and zircons from the Paleo- and Mesoarchean rocks of the Bulun terrane suggests that the latter are the most likely crustal sources of the granites. The more radiogenic isotope composition of the Paleoproterozoic (1.85 Ga) magmatic zircons from the granites as compared with the Archean crustal rocks of the Bulun terrane testifies to the contribution of juvenile material to the granite formation. Highly ferroan granodiorites and granites of the Shumikha pluton in the Onot terrane are enriched in HFSE and correspond to A-type granites. They probably derived by the melting of crustal sources of intermediate-felsic (tonalitic) and mafic composition at >860 °C. The Hf isotope composition of magmatic (1.86 Ga) and inherited (ca. 2.5 Ga) zircons indicates that the granites formed from ancient crustal source (model Hf age is >3.0 Ga) with the contribution of Neoarchean juvenile, probably mafic material.

Early Mesozoic deep-crust reworking beneath the central Lhasa terrane (South Tibet): Evidence from intermediate gneiss xenoliths in granites

Understanding the rheological behavior of the Tibetan Plateau and its response to geodynamic processes requires a clear knowledge of the composition, evolution and lithological properties of the deep crust. Here we present U–Pb–Hf isotopes of zircons, bulk-rock geochemistry and mineral compositions for seven intermediate gneiss xenoliths and their host Early Mesozoic granites (205±6Ma) in the central Lhasa terrane to probe the deep crust beneath Southern Tibet. The xenoliths contain plagioclase, amphibole, titanite, allanite, quartz, biotite and muscovite, with accessory Fe–Ti oxides, apatite and zircon. Bulk-rock and mineral geochemistry suggests that these xenoliths have a magmatic origin and experienced deformation and amphibolite-facies metamorphism (equilibration at pressures of 0.46–0.83GPa and temperatures of ~650°C), before they were captured by the host granite at ~205Ma. Zircons in these xenoliths show complex microstructures, including inherited cores, magmatic or metamorphic bands, and high U–Th hydrothermal rims. Inherited zircon cores record U–Pb ages from 2277Ma to 517Ma. Igneous zircons show a range of concordant U–Pb ages, suggesting a protracted magmatism from 236Ma to 203Ma. Metamorphic zircon zones record the timing of amphibolite-facies metamorphism from 224 to 192Ma, while the high U–Th hydrothermal rims show a subsequent fluid activity until ~150Ma. Unradiogenic Hf isotopic compositions of both xenoliths and host granites [xenolith εHf(t)=−11.2 to 0; host granite εHf(t)=−17.3 to −3.3] indicate that the Early Mesozoic deep crust in the central Lhasa terrane originated mainly from ancient (i.e., Proterozoic) crust, with little or no interaction with juvenile magmas. This study suggests a possible continental differentiation mechanism during crustal reworking; progressive melting may initiate from the lower mafic crust (at ca. 236Ma) and gradually migrate into the sediment-rich upper crust (until ca. 203Ma). The reworking results in the transition from small fluxes of intermediate magmas to voluminous peraluminous S-type granite in a convergent depth.

Paleoproterozoic granitoids of the Losevo terrane, East European Craton: Age, magma source and tectonic implications

The Losevo terrane lies between the Sarmatia and Volgo-Uralia segments of the East European Craton and is considered to be part of the East-Sarmatian Orogen (ESO). Here we report the results from field, petrologic, geochemical, Sm–Nd isotopic and zircon U–Pb geochronological studies on a suite of granitoids from the Losevo terrane (LT). The LT granitoids are divided into four types: (1) migmatite leucosome (from quartz diorite to granodiorite); (2) tonalite, trondhjemite and granodiorite (TTG) with high Si, Na, Sr contents, high Sr/Y, La/Yb ratios and low Mg, K, Y, Yb contents; (3) trondhjemite, granodiorite, granite (TGG) spatially related to the TTG suite, but showing higher K, negative Eu anomalies and flat REE patterns; and (4) high-K calc-alkaline monzogranite and granodiorite of I-type. The early (2115Ma) migmatites occur as lensoid and stromatic layers. The TTG and TGG (2100–2075Ma) constitute about 20% of the LT and form batholiths (150–540km2) and stocks. Small massifs of the I-type granites (2081±12Ma) intrude the migmatites. The migmatites, TTG and TGG contain inherited zircon cores that yield ages between 2130 and 2172Ma. The granitic suite shows positive εNd values ranging from +2.1 to +5.7 for the TTG and TGG and +2.1 and +2.2 for the migmatites, suggesting magma derivation from juvenile Paleoproterozoic sources (model ages=2255–2450Ma). In contrast, the I-type granite shows slightly negative εNd value of −0.5, suggesting that the magma sources involved a mixture of Archean and Paleoproterozoic components. All the granitoids from LT record varying degrees of fractional crystallization with or without crustal contamination. The temperature of parental melts of the granites are estimated to be in the range of 793±33°C to 878±16°C. The migmatites were derived mainly from metaigneous-sedimentary protoliths under P∼5–6kbar; whereas the TTGs were derived from lower crustal metabasites under P∼15kbar. The TGG were derived from mid crustal metagreywackes under P<10kbar, and the I-type granites were mainly derived from ancient lower crustal components involving amphibolites, gneisses and metagreywackes under P⩾12kbar. Our new data suggest that the anatexis and formation of migmatites occurred during the peak collisional event, and the other members of the granitoid suite including the TTGs formed during the post-collisional orogenic collapse of the ESO. The estimated average exhumation rate of the granitic suite during collision to post-collisional collapse is about 235m/Ma.

Contribution of Columbia and Gondwana Supercontinent assembly- and growth-related magmatism in the evolution of the Meghalaya Plateau and the Mikir Hills, Northeast India: Constraints from U-Pb SHRIMP zircon geochronology and geochemistry

The Meghalaya Plateau and the Mikir Hills constitute a northeastern extension of the Precambrian Indian Shield. They are dominantly composed of Proterozoic basement granite gneisses, granites, migmatites, granulites, the Shillong Group metasedimentary cover sequence, and Mesozoic-Tertiary igneous and sedimentary rocks. Medium to coarse grained, equigranular to porphyritic Cambrian granite plutons intrude the basement granite gneisses and the Shillong Group. U-Pb SHRIMP zircon geochronology and geochemistry of the granite gneisses and granites have been carried out in order to understand the nature and timing of granite magmatism, supercontinent cycles, and crustal growth of the Meghalaya Plateau and Mikir Hills. Zircons from the Rongjeng granite gneiss record the oldest magmatism at 1778±37Ma. An inherited zircon core has an age of 2566.4±26.9Ma, indicating the presence of recycled Neoarchaean crust in the basement granite gneisses. Zircons from the Sonsak granite have two ages: 523.4±7.9Ma and 1620.8±9.2Ma, which indicate partial assimilation of an older granite gneiss by a younger granite melt. Zircons from the Longavalli granite gneiss of the Mikir Hills has a crystallization age of 1430.4±9.6Ma and a metamorphic age of 514±18.6Ma. An inherited core of a zircon from Longavalli granite gneiss has an age of 1617.1±14.5Ma. Zircons from younger granite plutons have Cambrian mean ages of 528.7±5.5Ma (Kaziranga), 516±9.0Ma (South Khasi), 512.5±8.7Ma (Kyrdem), and 506.7±7.1Ma and 535±11Ma (Nongpoh). These plutons are products of the global Pan-African tectonothermal event, and their formation markedly coincides with the later stages of East Gondwana assembly (570–500Ma, Kuunga orogen). The older inherited zircon cores (2566.4±26.9Ma, 1758.1±54.3Ma, 1617.1±14Ma) imply a significant role for recycled ancient crust in the generation of Cambrian granites. Thus the Meghalaya Plateau and Mikir Hills experienced major felsic magmatic episodes at ~1800Ma, ~1600Ma, ~1400Ma, and ~500Ma with recycling of Neoarchaean crust, and later contributions from Paleo-Mesoproterozoic granite gneiss sources. A 258±20Ma lower intercept age of the Rongjeng granite gneiss perhaps indicates a Permo-Triassic thermal imprint on the Meghalaya Plateau. The granite gneisses and granites have peraluminous to metaluminous compositions, and syn-orogenic to post-collisional affinities. We conclude that the orogenic history of the Meghalaya Plateau and the Mikir Hills records crustal growth of the Columbia and Gondwana supercontinents as noted in other Pan-African-Indian-Prydz-Brasiliano orogens.

Petrogenesis and tectonic implications of Permian post-collisional granitoids in the Chinese southwestern Tianshan, NW China

Permian porphyritic granite and leucogranite from the Kekesu and Muzhaerte Valleys in the southwestern (SW) Tianshan orogenic belt, NW China have been studied to decipher their petrogenesis and tectonic implications. For porphyritic granite in the Kekesu Valley, in situ LA-ICPMS zircon U–Pb dating yields crystallization ages of 295–291Ma. The granite is a high potassic calc-alkaline, slightly peraluminous type, enriched in large ion lithosphere elements (LILE) and light rare earth elements (LREE), but depleted in high field strength elements (HFSE). Zircon Hf isotopic analysis (zircon εHf(t) of −5.8 to −0.2, two-stage Hf model ages of 1323–1680Ma) and Ti-in-zircon thermometry, which yields crystallization temperatures of 744–749°C, indicate the parent magma was likely formed by partial melting of a Mesoproterozoic crustal source. By contrast, leucogranite in the Kekesu valley yields crystallization ages of 274–267Ma. It contains muscovite and garnet, has high silicon and potassium, and is strongly peraluminous. Multiple inherited zircon cores and low zircon crystallization temperatures (687–701°C), combined with negative zircon εHf(t) values (−7.0 to −4.0), indicate its parent magma was sourced from supracrustal metasedimentary rocks by muscovite–breakdown partial melting. In the Muzhaerte Valley, porphyritic granite has similar major and trace elements characteristics to the Kekesu porphyritic granite. However, its higher zircon εHf(t) values (−0.9 to +3.8) and corresponding lower two-stage Hf model ages (1070–1367Ma) indicate that the parent magma likely included an input from a more juvenile mantle source. Ti-in-zircon thermometry gives lower crystallization temperature of ∼705°C. The intrusive relationships between the Permian granitoids and Paleozoic arc plutons, and the LP-HT and (U)HP metamorphic belts, combined with geochronological studies, suggest that these Permian granitoids were generated in a post-collisional environment. It is suggested that the formation of the porphyritic granites in the Kekesu and Muzhaerte Valleys could be related to slab breakoff, while the leucogranite could be formed by hydrate-breakdown melting in an extensional tectonic regime in response to intra-continental adjustments after collision.

The role of changing geodynamics in the progressive contamination of Late Cretaceous to Late Miocene arc magmas in the southern Central Andes

The tectonic and geodynamic setting of the southern Central Andean convergent margin changed significantly between the Late Cretaceous and the Late Miocene, influencing magmatic activity and its geochemical composition. Here we investigate how these changes, which include changing slab-dip angle and convergence angles and rates, have influenced the contamination of the arc magmas with crustal material. Whole rock geochemical data for a suite of Late Cretaceous to Late Miocene arc rocks from the Pampean flat-slab segment (29–31 °S) of the southern Central Andes is presented alongside petrographic observations and high resolution age dating. In-situ U–Pb dating of magmatic zircon, combined with Ar–Ar dating of plagioclase, has led to an improved regional stratigraphy and provides an accurate temporal constraint for the geochemical data.A generally higher content of incompatible trace elements (e.g. Nb/Zr ratios from 0.019 to 0.083 and Nb/Yb from 1.5 to 16.4) is observed between the Late Cretaceous (~72Ma), when the southern Central Andean margin is suggested to have been in extension, and the Miocene when the thickness of the continental crust increased and the angle of the subducting Nazca plate shallowed. Trace and rare earth element compositions obtained for the Late Cretaceous to Late Eocene arc magmatic rocks from the Principal Cordillera of Chile, combined with a lack of zircon inheritance, suggest limited assimilation of the overlying continental crust by arc magmas derived from the mantle wedge. A general increase in incompatible, fluid-mobile/immobile (e.g., Ba/Nb) and fluid-immobile/immobile (e.g., Nb/Zr) trace element ratios is attributed to the influence of the subducting slab on the melt source region and/or the influx of asthenospheric mantle.The Late Oligocene (~26Ma) to Early Miocene (~17Ma), and Late Miocene (~6Ma) arc magmatic rocks present in the Frontal Cordillera show evidence for the bulk assimilation of the Permian–Triassic (P–T) basement, both on the basis of their trace and rare earth element compositions and the presence of P–T inherited zircon cores. Crustal reworking is also identified in the Argentinean Precordillera; Late Miocene (12–9Ma) arc magmatic rocks display distinct trace element signatures (specifically low Th, U and REE concentrations) and contain inherited zircon cores with Proterozoic and P–T ages, suggesting the assimilation of both the P–T basement and a Grenville-aged basement. We conclude that changing geodynamics play an important role in determining the geochemical evolution of magmatic rocks at convergent margins and should be given due consideration when evaluating the petrogenesis of arc magmas.

Late Ediacaran crustal thickening in Iran: Geochemical and isotopic constraints from the ~550 Ma Mishu granitoids (northwest Iran)

ABSTRACTThe aim of this article is to examine the geochemistry and geochronology of the Cadomian Mishu granites from northwest Iran, in order to elucidate petrogenesis and their role in the evolution of the Cadomian crust of Iran. The Mishu granites mainly consist of two-mica granites associated with scarce outcrops of tonalite, amphibole granodiorite, and diorite. Leucogranitic dikes locally crosscut the Mishu granites. Two-mica granites show S-type characteristics whereas amphibole granodiorite, tonalities, and diorites have I-type signatures. The I-type granites show enrichment in large-ion lithophile elements (e.g. Rb, Ba and K) and depletion in high field strength elements (e.g. Nb, Ti and Ta). These characteristics show that these granites have been formed along an ancient, fossilized subduction zone. The S-type granites have high K, Rb, Cs (and other large ion lithophile elements) contents, resembling collision-related granites. U–Pb zircon dating of the Mishu rocks yielded 238U/206Pb crystallization ages of ca. 550 Ma. Moreover, Rb–Sr errorchron shows an early Ediacaran age (547 ± 84 Ma) for the Mishu igneous rocks. The two-mica granites (S-type granites) show high 87Sr/86Sr(i) ratios, ranging from 0.7068 to 0.7095. Their ɛNd values change between −4.2 and −4.6. Amphibole granitoids and diorites (I-type granites) are characterized by relatively low 87Sr/86Sr(i) ratios (0.7048–0.7079) and higher values of ɛNd (−0.8 to −4.2). Leucogranitic dikes have quite juvenile signature, with ɛNd values ranging from +1.1 to +1.4 and Nd model ages (TDM) from 1.1 to 1.2 Ga. The isotopic data suggests interaction of juvenile, mantle-derived melts with old continental crust to be the main factor for the generation of the Mishu granites. Interaction with older continental crust is also confirmed by the presence of abundant inherited zircon cores. The liquid-line of descend in the Harker diagrams suggests fractional crystallization was also a predominant mechanism during evolution of the Mishu I-type granites. The zircon U–Pb ages, whole rock trace elements, and Sr–Nd isotope data strongly indicate the similarities between the Mishu Cadomian granites with other late Neoproterozoic–early Cambrian (600–520 Ma) granites across Iran and the surrounding areas such as Turkey and Iberia. The generation of the Mishu I-type granites could be related to the subduction of the Proto-Tethyan Ocean during Cadomian orogeny, through interaction between juvenile melts and old (Mesoproterozoic or Archaean) continental crust. The S-type granites are related to the pooling of the basaltic melts within the middle–upper parts of the thick continental crust and then partial melting of that crust.

UHT granulites of the Highland Complex, Sri Lanka II: Geochronological constraints and implications for Gondwana correlation

The regional ultrahigh-temperature (UHT) metamorphism of the Highland Complex, Sri Lanka is well established and has an important role in our understanding of the tectonic history of the Gondwana supercontinent. U-Pb zircon dating of sapphirine-bearing Mg-Al granulites yielded two major metamorphic age populations at approximately 620-590 and 563-525 Ma with no evidence of older zircon cores. Pelitic granulite samples with a Grt-Sil-Spl-Crd assemblage have similar metamorphic ages with concordant data clusters at similar to 602, 563, and 526 Ma and inherited zircon cores aged from 2040 to 1600 Ma. The pelitic granulites also underwent two stages of metamorphism (565-520 and 622-580 Ma). Some of these pelitic granulite samples have inherited zircon cores ranging from 3060 to 760 Ma. Zircons in mafic granulite samples have age ranges of 566-533 and 620578 Ma. A calc-silicate granulite sample also has similar age populations at 591, 541, and 524 Ma. Combining these new results with previously published ages from Sri Lanka and formerly adjacent continental fragments of Gondwana, we propose that the terranes in southern Madagascar (south of Ranotsara Shear Zone), Northern and Southern Madurai and the Trivandrum Blocks of southern India, the Highland Complex of Sri Lanka, and the Skallen Group in the Lutzow-Holm Complex of East Antarctica represent a unique metamorphic belt that regionally experienced the Ediacaran-Cambrian UHT event during the amalgamation of the Gondwana super-continent.

Two-types of Early Cretaceous adakitic porphyries from the Luxi terrane, eastern North China Block: Melting of subducted Paleo-Pacific slab and delaminated newly underplated lower crust

The origin and tectonic setting of Early Cretaceous adakitic rocks from the Luxi terrane in the eastern North China Block (NCB) remain debated. To resolve this issue, we determined whole-rock geochemistry, zircon U–Pb ages, and in situ Hf–O isotopes of the Mengyin and Liujing adakitic porphyries from the Luxi terrane. Zircon U–Pb dating results reveal that both the Mengyin and Liujing plutons were emplaced during the Early Cretaceous, with weighted mean 206Pb/238U ages of 130±1Ma (2σ) and 131±2Ma (2σ), respectively. In addition, abundant Neoarchean–Paleoproterozoic inherited zircon cores are identified in the Mengyin adakitic porphyry with 207Pb/206Pb ages ranging from 2.53 to 2.42Ga. Rocks of both plutons are silicic (SiO2=65.4–70.2wt.%), metaluminous, and alkaline in composition, comprising mainly quartz syenite porphyries. Samples from both plutons are enriched in large ion lithophile elements (LILEs) (e.g., Rb, Sr, and Ba), and light rare earth elements (LREEs), depleted in high field strength elements (HFSEs) (e.g., Nb, Ta, and Ti), and heavy rare earth elements (HREEs), and have either positive or no Eu anomalies. In addition, both adakitic porphyries have high Mg# values (51–64), high Sr and La contents, low Y and Yb contents, and high Sr/Y (Mengyin=149–264; Liujing=58–110) and (La/Yb)N (Mengyin=32.4–45.3; Liujing=43.8–53.1) ratios, similar to adakitic rocks worldwide. The Mengyin adakitic porphyry has higher whole-rock εNd(t) values (–5.8 to −4.1), more radiogenic Pb [(206Pb/204Pb)i=18.35–18.39, (207Pb/204Pb)i=15.55–15.56, (208Pb/204Pb)i=38.20–38.23], higher zircon rim εHf(t) values (+3.3 to +8.8) and δ18O values (+6.5‰ to +7.9‰), and lower (87Sr/86Sr)i ratios (0.7049–0.7050) than the Liujing adakitic porphyry [εNd(t)=−12.4 to −12.2, (206Pb/204Pb)i=17.63–17.72, (207Pb/204Pb)i=15.56–15.58, (208Pb/204Pb)i=37.76–37.94, εHf(t)=−14.8 to −11.2, δ18O=+5.9‰ to +7.1‰, (87Sr/86Sr)i=0.7090–0.7091]. The Mengyin adakitic porphyry was most likely derived from partial melting of subducted oceanic slab with some input of NCB Neoarchean–Paleoproterozoic lower crust components. The Liujing adakitic porphyry was probably derived from partial melting of delaminated newly underplated thick lower crust, which then interacted with above asthenospheric mantle peridotite. Slab rollback together with the ridge subduction of the Paleo-Pacific slab was the most likely geodynamic mechanism for formation of the Early Cretaceous Mengyin and Liujing adakitic porphyries.