Assemblage Of Biotite Research Articles

Zircon U-Pb age and Hf isotopic composition of the Carboniferous Gönen granitoid in the western Sakarya Zone of Turkey

The Gonen granitoid is exposed in the western Sakarya Zone and is overlain unconformably by a Jurassic succession. The medium to coarse-grained Gonen granitoid has mineral assemblage of K-feldspar, plagioclase, quartz, muscovite, and biotite. Accessory phases are apatite and zircon. In this study, zircon U-Pb age is combined with Lu-Hf isotopes, which are presented to reveal the magma source and possible petrogenetic processes that took place during the formation of the parental magma for the Gonen granitoid. U-Pb dating of magmatic zircons yielded a concordia age of 336.3 ± 2.9 Ma referring to the early Carboniferous crystallization age of the Gonen granitoid. Magmatic zircons have negative eHf t values -3.2 to -8.3 , indicating that the granitoid magma was derived from the recycling of ancient crustal materials. TDM model ages vary in the range of 1489-1811 Ma, indicating that the crustal material involved during the early Carboniferous partial melting could be extracted from the mantle or added to the basement of the Sakarya Zone in the Mesoproterozoic/Paleoproterozoic times. Geochronological and Lu-Hf findings point to a collisional setting rather than ongoing subduction during the formation of the early Carboniferous Gonen granitoid.

Zircon U–Pb geochronology and clockwise P–T evolution of garnet-bearing migmatites from the Qinling complex in the Weiziping area of the Qinling Orogen, Central China: Implications for thermal relaxation after crustal thickening

Zircon U–Pb dating and phase equilibrium modeling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system were carried out to garnet-bearing migmatites from the Qinling Complex in the Weiziping area of the Qinling Orogen, Central China, to constrain their metamorphic P–T path and elucidate the tectonic setting and geodynamic mechanism of Early to Middle Devonian high-temperature (HT) metamorphism in the North Qinling Belt. The mesosomes of the migmatites are composed of garnet-bearing biotite–hornblende–plagioclase and biotite–plagioclase gneisses, and the leucosomes can be divided into two types: wide leucosomes that were deformed, and narrow penetrative leucosomes. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb analyses yielded weighted mean 206Pb/238U ages of 398.8 ± 4.7, 394.6 ± 2.6, 399.5 ± 7.3, 372.5 ± 4.2, 370.8 ± 4.3, and 368.5 ± 4.2 Ma for metamorphic zircons within the sampled mesosomes, and a weighted mean age of 408.7 ± 5.7 Ma for igneous zircons within a deformed leucosome. Based on the published data and the results in this study, the timing of granulite-facies metamorphism is constrained to be ca. 409–395 Ma. The mesosome sample (garnet-bearing biotite–hornblende–plagioclase gneiss) has a peak mineral assemblage of garnet, biotite, plagioclase, hornblende, quartz, ilmenite, and melt, which equilibrated at P–T condition of 8.5–9.7 kbar at 745–820 °C as constrained by isopleths of XAn and XPy in a P–T pseudosection, suggesting that these gneisses underwent medium-pressure granulite-facies metamorphism. Conventional garnet–biotite–plagioclase–quartz (GBPQ) geothermobarometry yielded P–T conditions of 703 °C at 10 kbar and 685–748 °C at 7.1–8.9 kbar, which correspond to prograde and retrograde metamorphic stages, respectively. On the basis of these P–T results, the studied samples document a clockwise P–T path characterized by temperature increase and pressure decrease during the prograde stage, and temperature decrease and pressure decrease during the retrograde stage. Ti-in-zircon thermometry yielded a temperature range of 781–793 °C for metamorphic zircons from the mesosomes, further demonstrating that they underwent granulite-facies metamorphism at ca. 409–395 Ma. The prograde metamorphic stage characterized by temperature increase and pressure decrease suggests that thermal relaxation after crustal thickening has induced the partial melting and migmatization. Thus, the crustal thickening that may be caused by continental collision between the North China Block and the South China Block should occur no later than ca. 409 Ma.

The eruptive chronology of the Yucamane-Calientes compound volcano: A potentially active edifice of the Central Andes (southern Peru)

We reconstruct the eruptive chronology of the Yucamane–Calientes compound volcano in southern Peru based on extensive fieldwork and a large dataset of geochronological (K-Ar, 40Ar/39Ar, U-Pb, and 14C) and geochemical (major and trace element) analyses. This compound volcano is composed of two edifices that have experienced discontinuous volcanic activity from the middle Pleistocene to the Holocene. The Calientes volcano has been constructed in four successive stages: Calientes I is composed of andesitic lava flows dated at ~500 ka. Subsequently, the Callazas ignimbrite (Calientes II stage) was emplaced ~160–190 ka, followed by the main cone-building stage (Calientes III) at ~130–100 ka. Finally, the Holocene Calientes domes were emplaced and represent the last eruptive products of this edifice. The Yucamane volcano has been constructed in three stages: Yucamane I consists of a succession of andesitic lava flows exposed at the base of the volcano that are older than 40 ka. Yucamane II (~36–30 ka) comprises a thick sequence of block-and-ash-flow deposits that represents an episode of dome growth predating the younger Yucamane cone (Yucamane III) built after 20–25 ka. During the Holocene, Yucamane vulcanian to sub-Plinian activity has emplaced tephra-fall and pyroclastic-density-current deposits. The most recent explosive eruptions occurred ca. 3000 BP and emplaced a tephra-fall and pumice-flow deposits. Most samples from Calientes volcano are andesites and dacites (60.1–67.7 wt% SiO2), while rocks from Yucamane volcano are basaltic-andesites to dacites (53.4–66.9 wt% SiO2). The rocks have a mineral assemblage of plagioclase, amphibole, biotite, orthopyroxene, clinopyroxene, olivine, and Fe-Ti oxides. The analyzed samples are categorized within a high-K, calc-alkaline series. Calientes volcano erupted mostly andesitic magmas, but its history is punctuated by rare eruptions of silica-rich magmas. In contrast, Yucamane volcano follows a different trend characterized by a gradual decrease in silica content through post-glacial time, from the large (VEI 3) sub-Plinian andesitic eruption of ~3 ka to moderate (VEI ≤ 2) vulcanian eruptions of basaltic-andesitic. On the basis of such recurrent and recent (Holocene), low-to-moderate explosive activity, Yucamane must be considered an active and potentially threatening volcano, which may affect the province of Candarave with ~8000 inhabitants.

Middle Eocene high-K acidic volcanism in the Princes’ Islands (İstanbul) and its geodynamic implications

The rock assemblages of the Princes? Islands, which are located to the south of mainland Istanbul, are regarded as parts of the Lower Paleozoic quartz sandstones, although they were initially considered as volcanic rocks by Swan in 1868. They differ from quartz sandstones by their vesicular texture and are devoid of any stratigraphic layering. Their mineral constituents are plagioclase (30%-35%), feldspar (35%-40%), and quartz (20%-25%), corresponding to rhyolite. The crystallization age of the rhyolites is 45.66 ± 0.84 Ma on the basis of the U-Pb zircon data. They show high-K calc-alkaline affinity. On primitive-normalized spider diagrams, negative anomalies of Ba, Nb, Sr, P, and Ti and positive anomalies of Pb are noteworthy. Their chondrite-normalized REE patterns are characterized by strongly fractionated patterns with demonstrative negative Eu anomaly, whereby middle REE are not fractionated relative to the heavy REE. These geochemical features suggest a fractionating mineral assemblage of feldspar, apatite, and biotite without significant involvement of garnet. The Lutetian rhyolites of the Princes' Islands are a part of the Middle Eocene magmatic associations of the West Pontides, related to collision of the Menderes-Taurus block with the Pontides.

Pre-eruptive magmatic processes associated with the historical (218 ± 14 aBP) explosive eruption of Tutupaca volcano (southern Peru)

Magma recharge into a differentiated reservoir is one of the main triggering mechanisms for explosive eruptions. Here we describe the petrology of the eruptive products of the last explosive eruption of Tutupaca volcano (southern Peru) in order to constrain the pre-eruptive physical conditions (P-T-XH2O) of the Tutupaca dacitic reservoir. We demonstrate that prior to the paroxysm, magma in the Tutupaca dacitic reservoir was at low temperature and high viscosity (735 ± 23 °C), with a mineral assemblage of plagioclase, low-Al amphibole, biotite, titanite, and Fe-Ti oxides, located at 8.8 ± 1.6 km depth (233 ± 43 MPa). The phenocrysts of the Tutupaca dacites show frequent disequilibrium textures such as reverse zonation, resorption zones, and overgrowth rims. These disequilibrium textures suggest a heating process induced by the recharge of a hotter magma into the dacitic reservoir. As a result, high-Al amphibole and relatively high-Ca plagioclase phenocryst rims and microlites were formed and record high temperatures from just before the eruption (840 ± 45 °C). Based on these data, we propose that the recent eruption of Tutupaca was triggered by the recharge of a hotter magma into a highly crystallized dacitic magma reservoir. As a result, the resident dacitic magma was reheated and remobilized by a self-mixing process. These magmatic processes induced an enhanced phase of dome growth that provoked destabilization of the NE flank, producing a debris avalanche and its accompanying pyroclastic density currents.

Tectonic implications of P-T paths derived for garnet-bearing felsic gneisses from the Dabie and Sulu ultrahigh pressure terranes, east-central China

Felsic gneisses usually represent the vast majority of rocks in high-pressure (HP) and ultrahigh-pressure (UHP) terranes, but the derivation of metamorphic pressure-temperature (P-T) paths for these rocks have been rarely undertaken. We studied a felsic paragneiss with the mineral assemblage of plagioclase, quartz, phengite, biotite, garnet, epidote, K-feldspar, and rutile from the Chinese Continental Scientific Drilling main hole (depth: 775 m), located in the Sulu UHP terrane, eastern China, by analyzing minerals with the electron microprobe and computing P-T pseudosections with PERPLE_X. Garnet includes phengite, feldspar, quartz, and rutile and retains a Ca-rich core (XCa = 0.42). Towards the rim Ca contents decrease (XCa = 0.36) and Fe and Mg contents increase. This zoning points to a prograde P-T path at pressures lower than 1.3 GPa. In combination with Zr-in-rutile thermometry and garnet volume isopleths in the P-T pseudosection, peak P-T conditions of ca. 1.2 GPa and 610 °C were constrained which are below the wet granite solidus. K-feldspar formation at the expense of phengite indicates a nearly isothermal decompression accompanied by late fluid infiltration. A similar P-T path was additionally derived for a published felsic orthogneiss from the Dabie UHP terrane using P-T pseudosections. The prograde P-T paths obtained in this study imply that not all felsic gneisses from the Dabie and Sulu UHP terranes underwent UHP metamorphism. This is explained by tectonic juxtaposition of various felsic gneisses from different depths in an exhumation channel.

Origin and significance of Early Miocene high‑potassium I-type granite plutonism in the East Anatolian plateau (the Taşlıçay intrusion)

The Early Miocene high-K I-type plutonic rocks constitute the early products of the Neogene to Quaternary magmatism, and the youngest exposed intrusions in the East Anatolian plateau. Here we deal with the petrogenesis of the Early Miocene Taşlıçay intrusion covering an area of ∼62 km2. The intrusion comprises leucogranite, and minor gabbro-monzodiorite and rhyolite porphyry. U-Pb dating of zircons from the leucogranite, monzodiorite and rhyolite porphyry yielded identical igneous crystallization ages of ∼19 Ma (Early Miocene). According to the modified alkali-lime index, the leucogranite and the rhyolite porphyry are alkali-calcic, while the gabbro-monzodiorite is transitional calcic to calc-alkalic. On variation diagrams, the gabbro-monzodiorite and the leucogranite as well as rhyolite porphyry form distinct bimodal groupings, whereby the leucogranite display well-defined linear differentiation trends, in contrast to the gabbro-monzodiorite. The leucogranite has relatively fractionated rate earth element (REE) patterns with concave-upward shapes and significant negative Eu anomalies; middle REEs are hardly fractionated with respect to heavy REEs. The gabbro-monzodiorite is characterized by high abundances of incompatible elements, slightly fractionated chondrite-normalized REE patterns with feebly negative Eu anomaly. The rhyolite porphyry is compositionally similar to the leucogranite. The geochemical features imply a fractionating mineral assemblage of hornblende, plagioclase and biotite for the leucogranite, and hornblende and ± plagioclase for the gabbro-monzodiorite. All the rock types display a narrow Sr and Nd isotopic variation (initial 87Sr/86Sr = 0.7053 to 0.7065; initial εNd = −0.5 to −3.8). The leucogranite and rhyolite porphyry exhibit gradually slightly higher initial 87Sr/86Sr and lower initial 143Nd/ 144Nd ratios relative to the gabbro-monzodiorite. Similarly, δ18O and initial εHf values of zircons suggest slightly increasing amount of crustal component from the leucogranite to the rhyolite porphyry. The gabbro-monzodiorite is probably related to partial melts from the slightly enriched lithospheric mantle. The magmas of the leucogranite and the rhyolite porphyry, on the other hand, probably resulted from the remelting of a middle- to high-K basic to intermediate rocks, compositionally similar to the gabbro-monzodiorite, and assimilated gradually slightly increasing amount of old high-silica crustal material. Several lines of evidence such as (i) presence of the well-developed dike swarm of rhyolite porphyry at the north eastern margin of the intrusion, (ii) exhumation of the intrusion at the earth’s surface by Middle Miocene, (iii) widespread apatite fission tract ages between 22 and 16 Ma from literature, and (iv) the absence of the exposed intrusions younger than the Early Miocene suggest that the Early Miocene represents probably a time of continental extension and exhumation in Eastern Anatolia and NW Iran.

Sapphirine-Bearing Pelitic Granulite from Ailaoshan Orogen, West Yunnan, China: Metamorphic Conditions and Tectonic Setting

To reveal the petrological characteristics, metamorphic evolution history and tectonic setting of the pelitic granulites from Ailaoshan Orogen, West Yunnan, China, a comprehensive study in mineral chemistry, petrogeochemistry and geochronology studies is presented in this paper. Two metamorphic stages of the granulites can be established: (1) the peak metamorphism recorded by the mineral assemblage of garnet, kyanite, K-feldspar and rutile, and the initial retrograde metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, spinel, K-feldspar, plagioclase and biotite; (2) the superimposed metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and ilmenite. Zircon LA-ICP-MS U-Pb dating indicates that the protolith of the granulite was deposited after 337 Ma. The initial retrograde metamorphism occurred at P-T conditions of 8.6–12 kbar at 850–920 oC estimated by mineral assemblages, the low pressure limit of kyanite stability and GBPQ geothermobarometer in Indosinian (about 235 Ma), and the late superimposed metamorphism occurred at the P-T condition of 3.5–3.9 kbar at 572–576 oC estimated by GBPQ geothermobarometer since 33 Ma. The first stage was related to the amalgamation of the South China and Indochina blocks during the Triassic, and the second stage was possibly related with the large scale sinistral slip-shearing since the Oligocene. It is inferred that the upper continental crust was subducted/underthrusted to the lower continental crust (deeper than 30 km) and underwent granulite-facies metamorphism and then quickly exhumed to the middle-upper crust (10–12 km) and initial retrograde metamorphism occurred due to the collision of the Indochina and South China blocks during Indosinian, which was followed by superimposition of the second stage of metamorphism since the Oligocene.

Deformation-enhanced HP-UHP metamorphic transformation of two-pyroxene granite in ductile shear zones at Yangkou, Sulu UHP metamorphic belt, China

We report that high strain deformation in ductile shear zones greatly enhanced high to ultrahigh pressure (HP-UHP) metamorphic transformation of a two-pyroxene granite in deep continental subduction zone. Integrated structural and petrologic study of rocks from the two-pyroxene granite block and shear zones in the block in the UHP shear complex at Yangkou, Sulu UHP metamorphic belt, reveals that the undeformed granite has preserved granitic texture and primary mineral assemblage of hypersthene, augite, plagioclase, K-feldspar, quartz and biotite. Metamorphic transformation of the granite is limited to grain boundaries due to low fluid activity and low reaction kinetics. In contrast rocks from the shear zones have a mylonitic texture and most of the primary minerals are transformed to HP-UHP phase minerals. Dislocation creep accommodated by dynamic recrystallization results in significant grain size reduction of the primary minerals in the mylonite, which increases reaction surface area and enhances metamorphic transformation. Fluid released by biotite transformation to HP-UHP minerals leads to diffusion mass transfer that greatly accelerates the metamorphic process in the shear zone. Deformation conditions are estimated ca 546 °C and pressure 2.5 GPa.

High-silica rhyolites of Niijima volcano in the northern Izu–Bonin arc, Japan: Petrological and geochemical constraints on magma generation and supply

Niijima volcano in the northern part of the Quaternary Izu–Bonin volcanic arc, Japan, consists predominantly of rhyolitic volcanic units associated with minor basaltic and andesitic units. Most of the rhyolites are characterized by a phenocryst assemblage of orthopyroxene (Opx), cummingtonite (Cum), biotite (Bt), and hornblende, which are classified into four types: Opx–Cum, Cum, Cum–Bt, and Bt (in eruption order). The major element compositions of the rhyolites are typified by high SiO2 (73–78 wt%) and K2O (1.5–3.3 wt%) contents, and the trace elements are indicative of a highly differentiated nature, which differs significantly from rhyolites at other sites in the Izu–Bonin arc. The presence of cummingtonite phenocrysts, and mineralogical and whole-rock chemical compositions of the rhyolite lavas clearly indicate that these magmas were produced under low temperatures (<800 °C) and low pressures (<3 kbar). The decreasing anorthite content (An mol.%) of plagioclase and Mg number (Mg#) of cummingtonite and biotite with time suggest progressive eruption from shallower (low-T and -P) magma chambers. The small volumes of coeval basaltic and andesitic volcanic units and mafic–intermediate magmatic inclusions in the rhyolitic lavas played an important role in rhyolite generation and evolution. The parental rhyolitic magmas were produced by partial melting of lower to middle crust by basaltic underplating, after which the magmas ascended and fractionated into >13 units in a shallow magma system. At the same time and at deeper levels, basaltic and rhyolitic parental magmas mixed to produce andesitic magmas between the basaltic and rhyolitic parental magmas. In some cases, basaltic and andesitic magmas in deeper level ascended and mingled with shallow rhyolitic magmas before eruption. The eruption of rhyolitic magmas from shallow levels might have been triggered by the injection of basaltic and andesitic magmas from deeper levels. The complex magmatic system beneath Niijima volcano formed within a relatively thick crustal sequence on the rear-arc side of the Izu–Bonin volcanic arc.

Cordierite-bearing granulites from Ihosy, southern Madagascar: Petrology, geochronology and regional correlation of suture zones in Madagascar and India

Madagascar, a major fragment of Gondwana, is mainly composed of Precambrian basement rocks formed by Mesoarchean to Neoproterozoic tectono-thermal events and recording a Pan-African metamorphic overprint. The Ranotsara Shear Zone in southern Madagascar has been correlated with shear zones in southern India and eastern Africa in the reconstruction of the Gondwana supercontinent. Here we present detailed petrology, mineral chemistry, metamorphic P–T constraints using phase equilibrium modelling and zircon U-Pb geochronological data on high-grade metamorphic rocks from Ihosy within the Ranotsara Shear Zone. Garnet-cordierite gneiss from Ihosy experienced two stages of metamorphism. The peak mineral assemblage is interpreted as garnet + sillimanite + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite, which is overprinted by a retrograde mineral assemblage of biotite + garnet + cordierite + quartz + plagioclase + K-feldspar + magnetite + spinel + ilmenite. Phase equilibria modelling in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) indicates peak metamorphic conditions of 850–960 °C and 6.9–7.7 kbar, and retrograde P–T conditions of <740 °C and <4.8 kbar, that define a clockwise P–T path. Near-concordant ages of detrital zircon grains in the garnet-cordierite gneiss dominantly exhibit ages between 2030 Ma and 1784 Ma, indicating dominantly Paleoproterozoic sources. The lower intercept age of 514 ± 33 Ma probably indicates the timing of high-grade metamorphism, which coincides with the assembly of the Gondwana supercontinent. The comparable rock types, zircon ages and metamorphic P–T paths between the Ranotsara Shear Zone and the Achankovil Suture Zone in southern India support an interpretation that the Ranotsara Shear Zone is a continuation of the Achankovil Suture Zone.

Characterization of anhydrous to hydrous paragenetic sequence from pyroxene-bearing and pyroxene-absent variants of the late Carboniferous Baobei pluton in west Junggar of China

The Baobei pluton in west Junggar, composed of an earlier pyroxene-bearing granitoid and a variety of later pyroxene-absent hornblende–biotite-bearing granitoids (quartz diorite, granodiorite, monzogranite), intrudes early Carboniferous volcano–sedimentary rocks. Zircons separated from the granodiorite provide a U–Pb age of 306.5 ± 2.6 Ma, which represents the age of magma emplacement. The pyroxene-bearing granitoid is porphyritic with phenocrysts of orthopyroxene and plagioclase. Two types of orthopyroxene (Mg-rich Opx-1, Fe-rich Opx-2) occur in the pyroxene-bearing granitoid. Both the Opx-1 and Opx-2 show depletions of light REE and enrichments of heavy REE with negative Eu anomalies. The evolved melt in equilibrium with the Opx-2 contains higher light REE concentrations compared with the parental melt in equilibrium with the Opx-1 based on geochemical modeling. The pyroxene-bearing and pyroxene-absent variants of the pluton record the progression from the anhydrous orthopyroxene–clinopyroxene assemblage to the hydrous hornblende–biotite assemblage. These rocks are high-K calc-alkaline, metaluminous to weakly peraluminous and ferroan, and have similar trace element compositions characterized by light REE enriched and constant heavy REE patterns with enrichments of Pb and depletions of Nb and Ta. The melt corresponding to different granitoids was formed by dehydration melting of basaltic rocks at 4.8–9.7 kbar and 1060–1146 °C within a post-collisional geological setting in west Junggar.

A ductile to brittle shear zone–hosted Cu mineralization: Geological, geochronological, geochemical and fluid inclusion studies of the Lingyun Cu deposit, southern Tianshan, NW China

The Lingyun Cu deposit is located at the northern margin of the South Tianshan Accretionary Complex, NW China. In the Lingyun mine area, the rocks are cut by a NW–trending Yanxingshan ductile to brittle shear zone. All orebodies are hosted in crystal tuff. The sulfide mineralization in the Lingyun Cu deposit can be divided into early (E) and late (L) paragenetic stages. The E-stage is characteristic of fault-controlled sulfide assemblages (chalcopyrite + bornite+chalcocite)–quartz veins in the foliation fabric of the crystal tuff with albite + sericite ± epidote ± biotite assemblages. In the L-stage, the assemblages of ankerite + chalcopyrite + bornite + chalcocite are present in veinlets that cut the E-stage assemblages. The sulfide assemblages in the Lingyun deposit are characterized by striations in the metamorphosed crystal tuff. Ribbon-like silicate minerals in the crystal tuff share common deformation features with the sulfide assemblages.The host rock of the Lingyun Cu deposit yields a concordia age of ca. 405 Ma, indicating that the Aerbishibulake Formation formed during southward subduction of the South Tianshan Ocean beneath the Tarim Block. Moreover, crystal tuff samples show well developed negative Sr anomaly and positive Rb, Th, U, Nd and P anomalies, which are common in arc-related volcanic rocks. The ca. 297 Ma Re–Os isochron age yielded by sulfides is much younger than the host rock, which implies that the Lingyun Cu deposit is a typical epigenetic deposit. Both similar REE fractionated patterns between sulfide ores and crystal tuff and the high initial 187Os/188Os ratio (18.54 ± 0.67) indicate significant crustal materials input. The 40Ar/39Ar plateau age of ca. 292 Ma from the ribbon-like sericite in crystal tuff is coeval with the mineralization occurrence in the Lingyun Cu deposit. By contrast, diorite dikes that crosscut the crystal tuff formed at ca. 221 Ma, which is much younger than the sulfide mineralization. Thus, the mineralization in the Lingyun deposit is likely to have been generated by shear zone activity rather than the intrusion of diorite dikes.Three types of fluid inclusions have been identified in the Lingyun Cu deposit: Aqueous (W-type), mixed aqueous–carbonic (M-type) and pure carbonic (C-type) inclusions. These three types of fluid inclusions are all found in the E-stage quartz whereas only the W-type fluid inclusions have been observed in L-stage ankerite. Fluid inclusions from the E-stage commonly contain CO2 and H2O, locally with N2 with low salinity, which are common in orogenic deposits. By contrast, L-stage fluid inclusions show low CO2 concentrations and salinity, indicating a significant inflow of meteoric water. Given the regional geology, ore geology, coeval ore-forming and deformation and fluid inclusion features, the Lingyun Cu deposit may be associated with the orogenic class of mineral systems.

Prolonged anatexis of Paleoproterozoic metasedimentary basement: First evidence from the Yinchuan Basin and new constraints on the evolution of the Khondalite Belt, North China Craton

Discerning the history of burial, residence at depth and exhumation of high grade metamorphic rocks from an orogen is essential to understanding crustal differentiation and orogenesis. Such high grade rocks are inferred to exist beneath the Yinchuan Basin, which is overlain by a thick covering of younger sediments, and is located at the inferred southwest boundary of the Paleoproterozoic Khondalite Belt in the northwestern North China Craton. The metamorphic basement sampled from a recently completed deep borehole in the Yinchuan Basin is migmatitic and contains an assemblage of garnet, biotite, sillimanite, plagioclase, quartz, minor K-feldspar and local occurrences of muscovite. Phase equilibrium modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O system constrains the peak P–T conditions of anatexis to 6–10kbar at 760–810°C, followed by final crystallization at 4.5–5.5kbar and 690–710°C. Ti-in-zircon thermometry for the narrow metamorphic zircon rims provides a temperature range of 687–753°C, demonstrating formation during retrogression to the solidus. NanoSIMS U–Pb analyses on the metamorphic overgrowths of zircon grains yields an upper intercept age of 1895±36Ma. Chronological and chemical analyses of monazite indicate irregular bright gray domains with relatively high Th and low Y and HREE contents developed during prograde metamorphism (1962±8Ma). Conversely, dark domains with low Th and high Y and HREE overgrew during retrograde cooling (1892±14Ma) that was synchronous with the development of metamorphic zircon rims. LA-ICP-MS U–Pb analyses of detrital zircons reveal that the metamorphic basement beneath the Yinchuan Basin has a sedimentary protolith of Paleoproterozoic age. The results directly confirm the existence of Paleoproterozoic basement beneath the Yinchuan Basin, and the well-constrained P–T–t path supports the involvement of the basement (as part of the western Khondalite Belt) in burial and exhumation processes during the 1.96–1.89Ga period, during which anatexis occurred for ∼70Ma.

P–T–t path and tectonic significance of pelitic migmatites from the Lüliang Complex in Xiyupi area of Trans-North China Orogen, North China Craton

In order to reveal the metamorphic evolution of the Lüliang Complex, a combined study of petrography, phase equilibria modeling and zircon U–Pb dating is carried out on two mesosome of pelitic migmatites from the Jiehekou Group in the Lüliang Complex. Sample (15XYP-02) preserved the thermal peak assemblage of garnet, biotite, K-feldspar, sillimanite, quartz and ilmenite, without plagioclase. It recorded two near-isothermal decompressional indicators that rutile was moated by ilmenite and garnet was replaced by cordierite. Small plagioclase occurred in the rim of K-feldspar and garnet. Four stages (M1–M4) of metamorphic evolution have been identified based on the mineral assemblages and P–T pseudosection. It is interpreted to record pre-Tmax decompression from rutile-stability field (M1, represented by rutile relicts in matrix and biotite+sillimanite+quartz inclusions in garnet porphyroblasts) at ∼8.9kbar to the Tmax stage (M2, characterized by garnet rim with ilmenite and biotite+K-feldspar+sillimanite+quartz in matrix) of 815–838°C. The following suprasolidus post-Tmax cooling comprises M3stage (represented by appearance of cordierite in the assemblage of garnet+biotite+K-feldspar+sillimanite+ilmenite+quartz) of 780–838°C/5.0–6.3kbar and M4stage (defined by the final mineral assemblage of garnet+biotite+K-feldspar+plagioclase+sillimanite+cordierite+ilmenite+quartz) of ∼775°C/∼5.0kbar. Another sample (15XYP-21) preserved the thermal peak assemblage of garnet, biotite, plagioclase, K-feldspar, sillimanite, quartz. It also records pre-Tmax decompression from rutile-stability field at 9.0kbar to the Tmax stage of 796–821°C/6.4–7.1kbar, followed by suprasolidus post-Tmax cooling stage of 778–803°C/5.0–5.7kbar. The clockwise P–T paths are consistent with those of orogenic belt. Furthermore, the zircon U–Pb dating on mesosome and leucosome of pelitic migmatite yield metamorphic ages of 1.95Ga and 1.91Ga, respectively, in which, the former represents the early stage of cooling after Tmax, nearly close to the time of pressure peak metamorphism. Combined with those granulites in the Trans-North China Orogen, a common long-lived continent-continent collision processes was proposed, which may culminated at ∼1.95Ga and followed by a prolonged cooling process.

Heavy mineral studies of Gondwana sandstones of Eastern Arunachal Himalaya and implications for provenance

A thin linear belt of Permian Lower Gondwana rocks occur in the Eastern Himalayas from Arunachal Pradesh to Sikkim. The Lower Gondwana Group of rocks consists of shale, siltstone, sandstone, carbonaceous shale and coaly matter and is sandwiched between Miri Group and Siwalik Group of rocks. Heavy mineral composition of sandstones is extensively used in the provenance studies as they are the surviving remnants of the rather abundant but unstable mafic components of the source rock. The sandstones of Lower Gondwana Group bear the heavy mineral assemblage of chlorite, biotite, zircon, ilmenite, epidote, garnet, amphibole, chloritoid, brown tourmaline, magnetite, staurolite, rutile, and opaques which is indicative of a provenance of metamorphic rocks with input of igneous rocks. Provenance sensitive mineral index ratios were also calculated to see the variation in the ratio of two or more stable minerals with the same characteristics.

Petrogenesis of incipient charnockite in the Ikalamavony sub-domain, south-central Madagascar: New insights from phase equilibrium modeling

Incipient charnockites representing granulite formation on a mesoscopic scale occur in the Ambodin Ifandana area of Ikalamavony sub-domain in south-central Madagascar. Here we report new petrological data from these rocks, and discuss the process of granulite formation on the basis of petrography, mineral equilibrium modeling, and fluid inclusion studies. The incipient charnockites occur as brownish patches, lenses, and layers characterized by an assemblage of biotite+orthopyroxene+K-feldspar+plagioclase+quartz+magnetite+ilmenite within host orthopyroxene-free biotite gneiss with an assemblage of biotite+K-feldspar+plagioclase+quartz+magnetite+ilmenite. Lenses and layers of calc-silicate rock (clinopyroxene+garnet+plagioclase+quartz+titanite+calcite) are typically associated with the charnockite. Coarse-grained charnockite occurs along the contact between the layered charnockite and calc-silicate rock. The application of mineral equilibrium modeling on the mineral assemblages in charnockite and biotite gneiss employing the NCKFMASHTO system as well as fluid inclusion study on coarse-grained charnockite defines a P–T range of 8.5–10.5kbar and 880–900°C, which is nearly consistent with the inferred P–T condition of the Ikalamavony sub-domain (8.0–10.5kbar and 820–880°C). The result of T versus H2O activity (a(H2O)) modeling demonstrates that orthopyroxene-bearing assemblage in charnockite is stable under relatively low a(H2O) condition of 0.42–0.43, which is consistent with the popular models of incipient-charnockite formation related to the lowering of water activity and stabilization of orthopyroxene through dehydration of biotite. The occurrence of calc-silicate rocks adjacent to the charnockite suggests that the CO2-bearing fluid that caused dehydration and incipient-charnockite formation might have been derived through decarbonation of calc-silicate rocks during the initial stage of decompression slightly after the peak metamorphism. The calc-silicate rocks might have also behaved as a cap rock that trapped CO2 infiltrated from an external source. ‘CO2-rich fluid ponds’ formed beneath calc-silicate layers could have enhanced dehydration of biotite to orthopyroxene, and produced layers of coarse-grained charnockite adjacent to calc-silicate layers.

Paleoproterozoic multistage metamorphic events in Jining metapelitic rocks from the Khondalite Belt in the North China Craton: Evidence from petrology, phase equilibria modelling and U–Pb geochronology

Metapelitic rocks of the Jining Complex (sillimanite-cordierite-garnet (Sil–Crd–Grt) gneisses, sillimanite-garnet (Sil–Grt) gneisses and quartzofeldspathic rocks) are exposed in the eastern segment of the Khondalite Belt (KB) in the North China Craton (NCC). The Sil–Crd–Grt gneisses have preserved polyphase mineral assemblages and microstructural evidence of anatexis, resulting from biotite dehydration melting. Petrological observations revealed that the Sil–Crd–Grt gneisses contain three metamorphic assemblages: a peak assemblage of garnet porphyroblast and matrix biotite+sillimanite+K-feldspar+plagioclase+quartz+ilmenite+magnetite, a post-peak near-isothermal decompressional assemblage of garnet+cordierite+biotite+sillimanite+K-feldspar+plagioclase+quartz+ilmenite+magnetite, and a decompressional cooling assemblage of garnet+biotite+cordierite+K-feldspar+plagioclase+quartz+ilmenite+magnetite. A clockwise P–T path was defined involving the inferred peak stage followed by post-peak near-isothermal decompression and decompressional cooling stages, with P–T conditions of 790–825°C and 9–10kbar, 810–890°C and 6.0–6.5kbar, and 780–810°C and 4.0–5.5kbar, respectively. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA–ICP–MS) U–Pb analyses of the Sil–Crd–Grt gneisses and Sil–Grt gneisses for the detrital and metamorphic zircons yielded a protolith age of ∼2.0Ga and the late Paleoproterozoic metamorphic age of 1895–1885Ma. The results reveal that the metapelitic rocks of the Jining Complex underwent continent–continent subduction or collision in the peak metamorphic stage, followed by a post-collisional exhumation event in the post-peak decompressional stage, and a subsequent decompressional cooling stage between the Yinshan and Ordos blocks to form the Paleoproterozoic KB.

Cogenetic origin of mafic microgranular enclaves in calc-alkaline granitoids: The Permian plutons in the northern North China Block

Mafic microgranular (microgranitoid) enclaves (MMEs) with fine-to medium-grain–size igneous microstructures are abundant in Permian granitoid plutons in the northern North China Block (NCB). They are mainly dioritic in composition with SiO 2 contents from 51.0 wt% to 62.7 wt% and are contiguous with their host granitoids (SiO 2 = 60.4–68.4 wt%). Their main mineral assemblage of plagioclase (oligoclase and andesine), hornblende, biotite, K-feldspar, and quartz is similar to that of their host granitoids but with different mineral proportions. Zircon laser ablation–inductively coupled plasma–mass spectrometer (LA-ICP-MS) U-Pb dating, hornblende-plagioclase thermobarometry, and Ti-in-zircon thermometry results show that the crystallization ages and pressure-temperature ( P-T ) conditions of the MMEs in each Permian granitoid pluton are very similar to those of their host granitoids, indicating simultaneous crystallization at the middle to upper crustal levels. Petrographical, geochemical, and Sr-Nd-Hf isotopic data show that the source areas of the Permian granitoid plutons in the northern NCB are temporally and spatially different and magma mixing was likely involved during formation of some granitoid plutons. However, the whole-rock Sr-Nd and zircon Hf isotopic compositions of the MMEs in each pluton are very similar to those of their host granitoids, indicating complete isotopic equilibration between the host granitoids and their enclaves. Combined with similar mineral assemblage and mineral chemistry between the MMEs and their host granitoids and absence of coeval mantle-derived rocks in or nearby the plutons, we confirm that the MMEs were crystallized from a coeval magma that gave rise to the host granitoids, and not by mixing and/or mingling between mantle-derived mafic and crustal-derived felsic magmas or by synplutonic injections of mafic magma into the host granitoid magma. We conclude that magma isotopic equilibration between host granitoids and their enclaves was achieved prior to emplacement and the magma mixing process during formation of some Permian granitoid plutons occurred mainly in the melting, assimilation, storage, and homogenization (MASH) zone in the lowermost crust or mantle-crust transition, not in the magma conduits or chambers at middle to upper crustal levels. A two-stage model that includes rapid cooling within the cogenetic host granitoid magma operating at pluton margins to form solid to sub-solid dioritic rocks and crystal accumulations of the host granitoid magma at the bottom of the pluton to form cumulates, and dioritic rocks and cumulates then fragmentated and interacted with progressive evolved host granitoid magma to change their shapes and orientations is proposed for origin of the MMEs in the Permian granitoid plutons in the northern NCB. This model may be more widely applicable to the generation of many MMEs in granitic rocks, especially where no direct evidence exists for the presence of mafic magmas coeval with granitoids and where there is a lack of isotopic contrast between hosts and enclaves. Our new results on origin of the enclaves in the granitoid plutons in the northern NCB show that some MMEs previously used as evidence for magma mixing and/or mingling are “autoliths (cognate xenoliths)” crystallized from the same magma as their host granitoids. There is an essential need for caution in using the MMEs as evidence for crustal-mantle interaction or mixing and/or mingling between crustal-derived felsic and mantle-derived mafic magmas during formation of granitic rocks, especially where no independent evidence is found for such a process. The role of MMEs in evaluating the mixing and/or mingling processes between crustal-derived felsic and mantle-derived mafic magmas has been much more likely overestimated by some previous studies and should be reconsidered. Instead, the MMEs in this origin can provide valuable information on emplacement mechanism and incremental growth of granitoid plutons in continental crust.

Bulk Composition of Mt. Mica Pegmatite, Maine, USA: Implications For the Origin of An Lct Type Pegmatite By Anatexis

Abstract The Mt. Mica pegmatite is famous for producing gem tourmaline for nearly 200 years. The dike, ranging in thickness from 1 to 8 m and dipping 20° SE, has a simple zonal structure consisting of a wall zone and core zone. The wall zone is essentially devoid of K-feldspar. The outer portion of the pegmatite consists of quartz, muscovite, albite (An 1.8), and schorl. Muscovite is the dominant K-bearing species in the outer portion of the pegmatite. Potassium feldspar only appears in the core zone adjacent to pockets. The pegmatite is subparallel to the foliation of the enclosing migmatite, and leucosomes show a gradational contact with the pegmatite where juxtaposed. Texturally, the pegmatite and leucosomes appear to be in equilibrium with no change in grain size or composition where the two are in contact. Garnet-biotite thermometry of the migmatite at the contact yields an average temperature of 630 °C, which is consistent with the pressure-temperature conditions inferred for a Sebago Migmatite Domain (SMD) assemblage of sillimanite, quartz, muscovite, biotite, and alkali feldspar formed at 650 °C and 3 kb. Gradational contact between the leucosomes and pegmatite suggests that the pegmatitic melt was at the same temperature. Coromoto Minerals LLC began mining in 2003 and the mine now extends down-dip for over 100 m to a depth of 33 m. A very detailed and accurately surveyed geologic map produced by owner/operator Gary Freeman during mining shows the total area of pegmatite removed, the spatial distribution and aerial extent of pockets, massive lepidolite (compositions near trilithionite) pods, microcline, and xenoliths. The map was analyzed using image analysis and thickness values of the units to calculate the total volumes of pegmatite mined, lepidolite pods, and all pockets found. Forty-five drill cores were taken across the pegmatite from the hanging wall to foot wall contacts, along a transect intentionally avoiding lepidolite pods and miaroles. Cores were pulverized, thoroughly mixed and homogenized, and the percent Li content calculated from the mapped volume was added to produce a sample that was representative of the bulk composition of Mt. Mica. The sample was then analyzed by fusion ICP spectroscopy for major and trace elements and DCP spectroscopy for B and Li. Structural water was determined by LOI. Water content was calculated using the calculated volume of open space (pocket volumes), assuming that the pockets were filled with water-rich fluid. This fluid content was added to LOI water (above 500 °C) to estimate a maximum H 2 O content of 1.16 wt.% of the pegmatite melt. REE plots of bulk pegmatite versus leucosomes from the migmatite are strikingly similar. Chondrite-normalized REE patterns of leucosomes and pegmatite are very flat with no Eu anomaly, whereas the Sebago granite is more strongly LREE-enriched and displays a pronounced negative Eu anomaly. Spider diagrams of leucosomes and pegmatite versus average crust show very similar patterns. These results suggest that the Mt. Mica pegmatitic melt did not form by fractional crystallization of the older Sebago pluton, but instead was derived directly from partial melting of the metapelitic rocks of the SMD. Batches of anatectic melt accumulated and coalesced into a larger volume that subsequently formed the pegmatite. This is the first chemical evidence presented for the formation of an LCT type pegmatite by direct anatexis.