Pure sediment-derived granites in a subduction zone

The Earth is unique in the Solar System due to significant volumes of granite in the lithosphere. However, the origins of granites are still highly debated, especially sediment-derived granites, which are often treated as a geochemical end-member of the continental crust. In the Yunnan region of South China, we identify the occurrence of pure sediment-derived granite in a subduction system. The suite of strongly peraluminous granite reported herein is interpreted to represent pure metasedimentary melts based on their whole-rock elemental and Sr-Nd-B and zircon Hf-O isotopic compositions. These Late Permian−Early Triassic (ca. 254−248 Ma) granites are characterized by radiogenically enriched Sr, Nd, and Hf isotopic signatures. They show δ11B and δ18O signatures akin to those of continental shales. Geochemical modeling indicates no contributions from the mantle that can be detected. Considering the regional tectonic evolution, these granites are suggested to be formed in a subduction zone by decompression melting of rapidly exhumed back-arc sediments. We posit that decompression melting was triggered by widespread extension and thinning of the crust prompted by rollback of the subducting oceanic crust. These granites thus provide evidence that granite formation in subduction zones does not necessarily contribute to crustal growth. These subduction-related pure sediment-derived granites have different elemental ratios and contents (e.g., Al2O3/TiO2 and Yb) from the Himalayan leucogranites. Considering their source compositions (e.g., pelitic rocks), which are similar to those of the Himalayan leucogranites, these differences are likely due to their higher formation temperature and lower pressure despite a great similarity in isotopic compositions. Identification of pure sediment-derived, strongly peraluminous granites (SPGs) in subduction systems provides an important geodynamic mechanism for crustal anatexis, which can both geochemically and tectonically complement their collisional counterparts identified in the Himalayas.

Multistage development of a hydrothermal tungsten deposit during the Variscan late-orogenic evolution: the Puy-les-Vignes W breccia pipe (Massif Central, France)

The Puy-les-Vignes W deposit, located in the northwestern French Massif Central (FMC), is a rare occurrence worldwide of a wolframite-mineralized hydrothermal breccia pipe hosted in high-grade metamorphic gneisses. We present here an integrated study of this deposit aiming to characterize the ore-forming hydrothermal system in link with the Variscan late-orogenic evolution of the FMC. Based on a set of representative samples from the host rocks and mineralization, we describe a detailed paragenetic sequence and we provide the major and trace element geochemistry of the host rocks and W-Nb-Ta-Sn-Ti oxide minerals, in situ U-Pb and 40Ar/39Ar geochronology, and a fluid inclusion study in quartz and wolframite. We demonstrate that the formation of this W-mineralized breccia pipe results from a multistage development between ca. 325 and 300 Ma related to four major episodes during the late Carboniferous. The first episode corresponds to the emplacement of an unexposed peraluminous granite at ca. 325 Ma, which generated microgranite dykes exposed at the present-day surface. The second episode is the formation of the quartz-supported breccia pipe and wolframite mineralization at ca. 318 Ma at a paleodepth of 7 km. The mineralizing fluids have a H2O-NaCl-CO2-CH4-N2 composition, a moderate-salinity (<9 wt.% NaCl eq) and were trapped at high-temperatures (400-450°C) during lithostatic to hydrostatic pressure variations caused by intense hydraulic fracturing of the host rocks. Wolframite deposition is interpreted to result from the mixing between a W-rich intermediate-density magmatic fluid exsolved from an evolved leucogranite and low-salinity volatile-rich metamorphic fluids of distal origin. The third episode corresponds to a magmatic-hydrothermal Nb-Ta mineralization overprinting the W-mineralized system and related to the intrusion at ca. 314 Ma of a rare-metal granite, which is part of a regional peraluminous rare-metal magmatism during the 315-310 Ma period. Finally, the last episode corresponds to the formation of a disseminated Bi±Au-Ag mineralization at ca. 300 Ma, which shares similar textural and mineral features with orogenic gold deposits in the FMC. The Puy-les-Vignes W deposit records, therefore, a multistage and long-lived development that extends over a timespan of 25 million years in a regional setting dominated by protracted peraluminous magmatism and HT-LP metamorphism. Although the local environment of ore deposition is atypical, our results show that the mineral assemblages, alteration styles, and fluid characteristics of the Puy-les-Vignes breccia pipe are similar to those of other peri-granitic W deposits in the FMC.

Zircon U–Pb geochronology, Hf isotope composition, and petrochemical characteristics of Paleocene granitoids in the western Gangdese Belt, Tibet

––The research team studied the petrology, whole-rock geochemistry, zircon U–Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to better understand the impact of Neo-Tethys ocean subduction and In-dia–Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO2, NaO2, and Al2O3 contents and low FeOT, MgO, and TiO2 contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet–peridotite partial melting and crust contamination from rising magma emplacement.

Tourmalinisation in peraluminous granitic context : from experiment to thermodynamic modelling

&lt;p&gt;Tourmaline records the physico chemical conditions during its cristallisation, as its primary chemical zonations are generally unbalanced, its occurrence as alteration product could be used to decipher the physicochemical properties of mineralizing fluids. However, the role of the tourmalinisation in hydrothermal processes remains little studied, if not poorly understood. &amp;#160;The complexity of its thermodynamic properties is related to the presence of four cationic sites allowing the accommodation of a wide variety of elements (Henry and Dutrow, 2018). Moreover the phenomena of deprotonation, Si-&lt;sup&gt;IV&lt;/sup&gt;B and valence state, make the approach of solid solution properties complex (Hughes et al., 2001; Henry et al., 2011; Ba&amp;#269;&amp;#237;k, 2015; Morgan, 2016). Thus, thermodynamic properties are most often estimated&amp;#160; (Garofalo et al., 2000; Hinsberg and Schumacher, 2007) and only a few measurements could be carried out on a reduced number of near-endmembers crystals (Kuyunko et al., 1984; Ogorodova et al., 2012).&lt;/p&gt;&lt;p&gt;This study aims to investigate experimentally the stability field of schorl (Na-Fe) &amp;#8211; dravite (Na-Mg) solid solution at 2 kbar total pressure between 400&amp;#176; and 600&amp;#176;C as a function of the boron content of the fluid and fO2 condition, using an internally heated gas apparatus. Those metasomatic experiments have been conducted on a mixture of naturals crystals of cordierite + albite, representing a peraluminous granite composition in a Na-Mg-Fe-Al-Si-B-O-H system, characterized by a high-Mg, low-Fe content. These experiments were performed in order to simulate a classic aluminous host of these tourmaline alterations in granitic context. The results will be studied, in terms of stability of the tourmaline species, chemistry variation and texture. They will be compared with thermodynamic models build using data from the literature (Korges et al., 2018; Pan et al., 2019 among others) . Ultimately, the objective is to characterize in a P, T, W/R space, the chemical evolution of fluids, the alteration sequence of rocks and the variations in volumes related to the successive reactions.&lt;/p&gt;

Timescales of plutonic-subvolcanic-volcanic connection in a Mio-Pliocene long-lived igneous system (Tuscany): zircon CA-ID-TIMS dating

&lt;p&gt;The genetic link between plutonic and volcanic realms is a key for understanding timescales of igneous plumbing systems, and precise geochronological records are pivotal in estimating the duration of processes at different levels in such plumbing systems. The Campiglia igneous complex, Tuscany, offers exposures of the full range of emplacement levels (plutonic, subvolcanic, volcanic) of mantle- and crust-derived magmas. Magma emplacement occurred astride the Miocene-Pliocene boundary. New high-precision U-Pb CA-ID-TIMS, zircon geochronological data, coupled with LA-HR-ICP-MS zircon dates for the whole Campiglia system define a short crystallization time span for zircon from the peraluminous granite pluton (~100 ka), intermediate for the shallow-level mafic porphyry (~450 ka), and longer for the rhyolite (~700 ka), at odd with what commonly expected. The oldest ages for the three units are the same, leading to hypothesize the occurrence of a bimodal deep reservoir remaining in melt-present conditions for some 700 ka. In this framework, early-crystallized zircons were cannibalized by younger melt batches that were sequentially extracted from the reservoir.&lt;/p&gt;

Petrology and Geochemistry of The Volcanic Arc Tarusan Pluton in Comparison to Lolo Pluton, West Sumatra

The Volcanic Arc Suite is the group of batholiths in the range of the Barisan Mountains and mostly denotes I-type affinity. Previous investigations of the intrusions in West Sumatra emphasized the crystallization age without completing geochemistry characteristics. No former study discussed a pluton which mapped in the Kota XI Tarusan District. This study explains the geochemistry and petrology of the Tarusan Pluton using polarized microscope, XRF, and ICP-MS at the Center for Geology Survey of Indonesia. The microscopic analysis confirms the granite character of the samples. Although both plutons are identified as I-type calc-alkaline series, the Tarusan Pluton is peraluminous granite whilst the Lolo Pluton denotes wider range from metaluminous to peraluminous of granodiorite to granite. Both the plutons are clearly classified as volcanic arc granitoid in the correlation to Volcanic Arc Suite of Sumatra. Negative Ba, Nb, and P anomalies together with positive K, Nd, and Y anomalies are pronounced on the two felsic intrusions. Negative Eu anomaly on the Tarusan Pluton but the positive one at the Lolo Pluton might explain different magma evolution process.Keywords: volcanic arc granite, geochemistry, Tarusan Pluton, Lolo Pluton.

Petrology and Geochemistry of The Volcanic Arc Tarusan Pluton in Comparison to Lolo Pluton, West Sumatra

The Volcanic Arc Suite is the group of batholiths in the range of the Barisan Mountains and mostly denotes I-type affinity. Previous investigations of the intrusions in West Sumatra emphasized the crystallization age without completing geochemistry characteristics. No former study discussed a pluton which mapped in the Kota XI Tarusan District. This study explains the geochemistry and petrology of the Tarusan Pluton using polarized microscope, XRF, and ICP-MS at the Center for Geology Survey of Indonesia. The microscopic analysis confirms the granite character of the samples. Although both plutons are identified as I-type calc-alkaline series, the Tarusan Pluton is peraluminous granite whilst the Lolo Pluton denotes wider range from metaluminous to peraluminous of granodiorite to granite. Both the plutons are clearly classified as volcanic arc granitoid in the correlation to Volcanic Arc Suite of Sumatra. Negative Ba, Nb, and P anomalies together with positive K, Nd, and Y anomalies are pronounced on the two felsic intrusions. Negative Eu anomaly on the Tarusan Pluton but the positive one at the Lolo Pluton might explain different magma evolution process.Keywords: volcanic arc granite, geochemistry, Tarusan Pluton, Lolo Pluton.