Grade Metamorphic Rocks Research Articles

A multi-proxy detrital study from Permian-Triassic fluvial sequences of Victoria Land (Antarctica): Implications for the Gondwanan basin evolution

One of the most complete Permian-Triassic fluvial sequences of the Beacon Supergroup, characterizing the infilling of the Transantarctic Basin, is recorded in the Allan Hills in Victoria Land (Antarctica). The multidisciplinary provenance approach carried out in this study includes UPb analysis on detrital zircons, chemistry on detrital mineral phases (garnet and white mica), and sandstone petrography. These, coupled with the data regarding the stratigraphic/sedimentological framework, provide a reconstruction of the geological history of the Permian-Triassic portion of the Beacon Supergroup, showing shifting clastic provenance from different source rocks. Results obtained from this integrated study support a source for the Permian Weller Coal Measures from the local basement, made up of very low- to high grade metamorphic rocks. A compositional shift is recorded with the Lower Triassic Feather Conglomerate, mainly linked with changes in the fluvial style, likely associated with tectonic activity. The Middle to Upper Triassic Lashly Formation shows the main provenance change, supporting a distal source region: the fluvial system received a volcanoclastic input derived from the Permian-Triassic arc located to the east of the basin. The compositional and provenance variations, together with paleocurrent indicators, compared to the coeval units in the central Transantarctic Mountains, allow the reconstruction of the basin evolution and of the changing clastic drainage patterns. Data support the occurrence of articulated sub-basins where alluvial sedimentation settled, in which a morphological-structural divide was represented by the Ross High, separating during the Permian up to the Early Triassic the Victoria Land sub-basin from the main Transantarctic Basin. According to the changed composition of the Lashly Formation sandstones, it appears that in the Middle/Late Triassic, the Ross High was not more a morphological barrier, and that the Victoria Land sub-basin was joined with the Transantarctic foreland Basin.

Texture and major element geochemistry of channel sediments in the Orsang and Hiren River Basins, Gujarat, India: Implications for provenance and weathering

Size, shape, degree of sorting, and composition of sediments in the river channels are controlled by climate, lithology, weathering, sorting, and medium of transportation. The present investigation is focused on the grain-size and geochemical analysis of the channel sediments of the Orsang and Hiren river basins. Major outcrops in the study area are Archaean granites, granitic gneisses, Upper Cretaceous to lower Eocene Deccan Volcanic Basalts (DVB), Quaternary sediments and minor proportion of Proterozoic low grade metamorphic rocks. The sediments are poorly to moderately sorted, very finely skewed, suggesting its derivation from heterogenous sources, while the kurtosis value indicates a high-energy depositional environment. The sediments are with gravelly sand texture and the mean grain size is varying from 581.9µm to 1284.2µm. The DVB provenance of the Hiren river basin and granitic provenance of the Orsang river basin is clearly reflected in the texture and geochemical composition of sediments. The TiO2 and Fe2O3 contents of sediments from the Hiren river basin are distinctly higher and are comparable to the basalts of the Saurashtra region of the Deccan Province. Sediments collected after Orsang and Hiren rivers confluence and from Narmada river show higher concentration of felsic sources, indicating that Orsang river's sediment supply significantly outweighs Hiren rivers. The arkosic-litharenite nature points towards less transportation and moderate chemical weathering for the Orsang river sediments. The low Chemical Index of Alteration (CIA) values (Avg. 48.45 and 56.99 for Orsang and Hiren rivers, respectively) and A-CN-K plot also suggest the supply of sediments from minimally weathered detritus under a semi-arid condition.

Late Cretaceous – Paleogene tectonothermal evolution of the Akdağ Massif in the central anatolian crystalline complex (northern Kayseri, central Turkey)

The Central Anatolian Crystalline Complex (CACC) is a Late Cretaceous continental crystalline basement exposed in central Turkey. Here, we report the metamorphic conditions and cooling history of the southern Akdağ Massif, which is the northeastern margin of the CACC. Thermobarometry performed on the garnet-sillimanite-bearing metapelitic rocks indicate ∼5-6 kbar/650-700 °C metamorphic conditions. Following HT-MP metamorphism, conditions of ∼3-4 kbar and ∼720 °C were also recorded by the metapelitic rocks. The replacement of sillimanite, cordierite, and biotite on the rims of garnet indicates LP-HT metamorphism. The HT decompression path described in this study is similar to conditions that have been previously proposed for the CACC. The LP-HT metamorphism resulted from the increased geothermal gradian arising from local granitoid intrusions, which are most likely related to Late Cretaceous subduction below the CACC, at ∼82-72 Ma. The Ar/Ar geochronology indicates that cooling of the high-grade rocks occurred at 75±0.8 Ma and the relatively younger Ar/Ar cooling ages of 61±0.5 Ma (amphibole) and 55.0±0.3 Ma (muscovite) were obtained from the medium/low grade metamorphic rocks of the Akdağ Massif. These cooling ages and stratigraphic evidence from the middle Eocene sedimentary cover, suggests a rapid exhumation and cooling rate of the Akdağ Massif is about ∼40 °C/Ma. We suggest therefore, that the exhumation of the Akdağ Massif was finalized by the Late Paleocene – Early Eocene and we suggest that this rapid exhumation can be explained by tectonic effects rather than by erosional alone.

Formation of authigenic grey monazite: A palaeo-thermal anomaly marker in very-low grade metamorphic rocks?

Reassessment of France’s rare earth element (REE) potential led us to investigate REE behaviour in the black shales of the Middle Ordovician Angers-Traveusot Formation in central Brittany (France). This study focuses on the distribution of nodular grey monazite (up to 200 g/t) within the shales, which formed in response to the diagenetic and low-grade metamorphic evolution of the studied area. Whole rock geochemistry, rock–eval pyrolysis and evolution of clay mineral crystallinity were firstly used to determine temperature and mass transfer conditions in the black shales. Then, monazite texture, composition and U–Pb in-situ dates were determined, and correlated with the diagenetic/anchimetamorphic conditions. Nodular monazite appears in the Ordovician black shales at the transition between high-grade diagenesis and the anchizone metamorphic facies, in response to processes controlled by competing influences such as organic matter maturation, Fe oxide/hydroxide reduction and clay transformation with accompanying fluid release. Monazite occurs mainly as elongated nodules, up to 2 mm in diameter that are characterised by their grey colour caused by an abundance of host-rock mineral inclusions. Monazite nodule compositions are zoned with Nd and middle REE-rich cores surrounded by Ce-rich rims, with no evidence of inherited domains. Ce-monazite also occurs as a replacement of nodular monazite or in late-stage fractures. Zoned nodular grey monazites were dated at ca. 403.6 ± 2.9 Ma, which is proposed to record the high heat flux that led to the anchimetamorphic conditions found at the base of the Angers-Traveusot formation, prior to the Variscan deformation. Crystallisation of the metamorphic Ce-monazite occurred at two periods, dated at 382.6 ± 2.9 Ma and 349.6 ± 6 Ma, which correspond to pre-collisional and collisional tectono-metamorphic stages respectively. Nodular grey monazite constitutes an interesting alternative economic solution because of its very low content in both Th (X¯=2160 ppm) and U (X¯=145 ppm) and negligible radiological impact if mined. However, placers currently display a limited economical interest.

Estimation of soil overburden thickness/depth of rock strata using geo-physical survey at Himalayan region


 
 
 Estimation of soil overburden and depth of competent strata for construction of infrastructures in hilly terrain is a challenging job for engineers. Modern technologies like Electrical Resistivity Survey (ERS) techniques with site geological mapping have been carried out at the ridge area near Palace, Gangtok. Two independent 2D-electrical resistivity surveys profile sections using hybrid Schlumberger-Wenner method were conducted along the ridge line trending in the North-South direction. The ERS profile sections were taken on both the slopes of the ridge along its eastern and western flanks. With the collective information from field and geo-technical data, a comparatively stable slope has been identified with respect to geological conditions in the present work, which focuses on the vulnerable slope failure with respect to slope direction, local geological condition, depth of competent strata, thickness of soil overburden, water saturation zones and resistivity of the materials. Diamond core drilling of 15meters each was carried out at both the flanks of the slope to understand the sub-surface strata and correlate it with resistivity data generated by ERS survey. The result indicates that Sandy/silty soil with flakes of mica having resistivity of 107 ohm-m and weathered mica schist having resistivity more than 300 ohm-m in the present study area. Geological mapping in 1:3000 scale was carried in the area demarcation with various litho-units and rock type. The area is characterized by medium grade metamorphic rock sequence represented in the area by mica schist having dip of foliations towards NE direction and three sets of joint planes. The geometry of the rock orientation and slope direction plays a vital role for determining the overall stability condition of the area. The present study will provide technical input for structural engineers to design the structures in such geological conditions. Further, the thickness of overburden estimated from ERS has been validated by drilling data.
 
 

Petrography of The Main Range Granite Characterization in Titiwangsa Rest Stop, Perak, Malaysia

The area of research is located in between the Titiwangsa Rest Stop, Perak allocated along the Gerik-Jeli highway, approximately 25km in area. The objective of this research is to remodify a geological map with a scale of 1:25,000 and to determine the petrological aspects of the metamorphic mineral assemblages around the study area. One of the methods used during the initial of this research is primarily on previous research and literature review. The geological mapping technique such as traversing was applied to map the allocated study area and rock samples are collected from three different outcrops that are exposed around the area as well. Moreover, the samples are then sent to a lab to undergo microscopic and petrographic analysis of the rocks’ compositions by producing thin sections to determine the mineral assemblages which surround the area on Titiwangsa Rest Stop, Perak. The rocks that can be identified around the area include igneous and metamorphic rocks such as biotite granite, gneiss, and mica-schist. The area of research which extends from the Titiwangsa Rest Stop towards the Gerik-Jeli highway is located directly on the Bentong-Raub suture zone. The depositional setting hypothesized in the area is a marine depositional coastal environment which includes a transition of a lower grade metamorphic rock towards a higher-grade metamorphic rock.

Signature of Himalayan orogenic features in Brahmaputra River sediments, Bangladesh: Evidence from single-grain heavy mineral chemistry

The present study describes results obtained from the chemistry of detrital heavy minerals i.e. pyroxene, amphibole, biotite, garnet, epidote and Fe-Ti oxides in fluvial sediments of the northern Brahmaputra River (Bangladesh) with an aim to determine conditions of their petrogenesis and provenance. The primary and secondary genera of ferromagnesian minerals occurred in calc-alkaline and peraluminous subduction zone. In which, the garnets are Fe-rich, indicating mostly almandine component (Alm65–Pyp16–Grs8–Sps6 averagely), occurred in medium to high grade metasedimentary rocks in the Lesser Himalaya (LH), along the Main Central Thrust (MCT) and the eastern Himalayan syntaxis. Besides, the fingerprint of omphacite and actinolite owe to ascertain the co-existence of garnet developed in ultrahigh-pressure (UHP) eclogites that may also be drained from the Tso Morari massif. Augite to aegirine-augite pyroxenes emphasizes Fe enrichment in basaltic systems and high to ultrahigh grade metamorphic rocks, which are exposed in the LH, Shillong Plateau, Mikir Hills, South Tibetan Detachment System (STDS), eastern Himalayan syntaxis and Tso Morari massif. Geochemistry and thermobarometry of the primary magmatic amphiboles and biotites manifest the source of granitoid and granodiorite like bodies, and their windows are exposed in the Bomi–Chayu, Gangdese arcs and the western Arunachal Himalaya. Again, metamorphosed Fe-Ti oxide minerals are well-exposed along the NE Lesser Himalaya, where magmatic derivative of Fe-Ti oxide minerals were modified through the diffusional processes in low-grade metamorphism (534–562 °C with 10–22.1–10−21.5fo2). Integrating the aforementioned discussion with the thermochronology, it is evident that the eastern Himalayan syntaxis is the major source of sediment flux, which is carried mostly by the upper Himalayan tributaries i.e. Yigong, Parlung, Dibang and Lohit. Also, the lower Himalayan tributaries i.e. Subansiri and Manas drain the sequestered derivatives dominantly from the Arunachal Himalayan. Tso Morari eclogites (NW Himalaya) have also contribution somewhat of dense minerals to the Tsangpo-Brahmaputra River system. Thus, scrutinizing the fingerprint of single-grain detrital minerals provides key information regarding the source terrains and tectonics of the Himalayan sequences.

Geochemistry of the Middle Eocene Gercus Sandstone, Shaqlawa Area, Northern Iraq: Implications for Provenance, Tectonic Setting and Paleoweathering

The study of the geochemical characterization of sandstone rocks contributes to understand the nature of the source rocks, their tectonic setting and the effects of paleoclimate and paleo-weathering. Accordingly, twenty sandstone samples from the red sandstone units of the Gercus Formation were collected from the northeastern limb of Safin Anticline at Shaqlawa Area – Northern Iraq. The studied sandstone samples show depletion in major, trace elements contents, except for CaO, MgO and Ni, Co and Cr respectively, and general depletion in the rare earth elements (REE) contents. Major element geochemistry indicates that the red bed sandstones of the Gercus Formation are of an oceanic island arc (OIA) tectonic setting. On the other hand, the results revealed that the paleo-weathering of the source area of the Gercus Formation experienced intensive chemical weathering under humid and/or warm climate conditions. The maturity and paleoclimate conclusions of the studied samples suggest a low feldspar content and low values of K2O / Al2O3 ratio, which in turn resulted from the dominance of low grade metamorphic and sedimentary rock types in the source area.

EPMA and PIXE Analyses of High Grade Metamorphic Rocks

Background/Objectives: The demand for exact quantification analyses in complex ore assemblages is increasing rapidly as mineral processing studies become more sophisticated. In response to this need, an Electron Micro Probe Analysis and Proton Induced X-ray Emission complementary based analytical tool procedure for the chemical characterization of minerals was employed to study the metamorphic rocks. Methods : In this work, we compare the quantitative analysis of high-grade metamorphic rocks by proton induced X-ray emission (PIXE) with that of the previous analyses by electron probe microanalysis (EPMA). During this investigation, the PIXE analytical method on a nuclear microprobe was added to the previously well-known EPMA for the chemical quantification of high-grade metamorphic rocks from the Charnockite Hill, Visakhapatnam, A.P., India. For the evolution and geochemical characterization of the metamorphic rocks, the above two methods are compared and discussions are made with respect to certain elements. Findings: Particle-induced X-ray emission is well-matched except for certain elements for the chemical characterization of metamorphic rocks belonging to earth material samples. This comparison with EPMA demonstrates the elemental quantification characteristics for such analyses. Not only is an exploratory study of proton induced X-ray emission benefit presented, but also a synopsis of comparison with electron probe microanalysis aimed at a responsible reply to metamorphic rock analytical questions. Novelty: The complementary technique, especially EPMA, is required to fill the elemental gaps that are not determined in the case of matrix complex charnockite compositional analysis by PIXE with 3MeV and a Si (Li) detector. Keywords: Electron and Proton Microprobes; Complementary Techniques; Metamorphic Rocks; Elemental Analysis

Petrological and chronological characteristics of medium-high grade metamorphic rocks in Gonjo area, southeastern margin of Qamdo block, eastern Tibet

青藏高原东缘的昌都地块是连接青藏高原腹地和藏东南三江-印支地区的关键区域, 发育有较完整的晚古生界, 以三叠系、侏罗系-白垩系广泛分布为特色; 但其前寒武纪基底岩系出露较少, 主要以宁多岩群(Pt_1-2<i>Nd</i>.)和草曲群(Pt₃<i>C</i>)为代表。本文针对昌都地块东南缘贡觉地区出露的一套被前人称为"宁多岩群"的中深变质岩系, 开展了系统的野外地质调查、显微薄片分析、锆石U-Pb年代学等研究工作, 认为该套中深变质岩系属于一套变质杂岩, 其岩石组合既包括了原岩沉积时代不早于奥陶纪的片麻岩、变粒岩和大理岩等副变质岩, 又包含了原岩时代分别为中二叠世的斜长角闪岩和中三叠世花岗质片麻岩等正变质岩。其原岩时代特征与昌都地块北缘的宁多岩群有较大差别, 不应当归属为昌都地块的前寒武纪结晶基底。结合片麻岩的变质新生云母类矿物Ar-Ar测年数据和变质后期侵入的闪长玢岩脉锆石U-Pb年龄, 认为贡觉地区出露的中深变质岩的变质时代为ca.260~244Ma(P₂-T₂)。该期发生在昌都地块东南缘的角闪岩相变质作用, 是金沙江古特提斯洋向西俯冲-碰撞的构造响应。

Formation of corundum in direct contact with quartz and biotite in clockwise &lt;i&gt;P–T&lt;/i&gt; trajectory from the Sør Rondane Mountains, East Antarctica

We have found corundum in direct contact with quartz and biotite and as inclusions in garnet in the pelitic gneisses of northern Austkampane in the Northeastern (NE) Terrane of the Sør Rondane Mountains (SRM), East Antarctica. Our samples, which include corundum–bearing gneisses, show petrographic features such as staurolite inclusions in garnet, compositional zoning of orthoamphibole with Al decreasing toward the rims, and late–stage cordierite formation, and these features are characteristic of a clockwise P–T trajectory. The observations are consistent with the proposed regional clockwise P–T evolution of the NE Terrane in the SRM. The corundum and other inclusions observed in the garnet porphyroblasts are interpreted to have formed owing to either staurolite breakdown or metastable crystallization relative to kyanite prior to the peak metamorphism. The close association of biotite and quartz surrounding corundum inclusions suggests fluid– or melt–related processes. These petrographic features imply that the corundum and quartz (rarely observed in high–grade metamorphic rocks) formed as a result of metastable crystallization during the prograde stage of the clockwise P–T evolution of a continental collision zone.

Dating Continental Subduction Beneath the Samail Ophiolite: Garnet, Zircon, and Rutile Petrochronology of the As Sifah Eclogites, NE Oman

AbstractStudies of the high‐pressure (HP) As Sifah eclogites in the NE Saih Hatat Window, Oman, have used different combinations of radiometric dating results (Ar/Ar, Sm‐Nd vs. U‐Pb, Rb‐Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction—including its relationship to the Samail Ophiolite. To determine the timing of continental subduction, we coupled petrochronological analyses of major (garnet) and accessory phases (zircon and rutile) from the highest grade metamorphic rocks (As Sifah eclogites) in the Saih Hatat. Early Permian (283.8 ± 0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions and metamorphic assemblages consistently suggest a single metamorphic event with garnet growth from ∼81 to 77 Ma—though with slight, consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous interpretations for continental HP metamorphism in a single NE‐dipping subduction zone beneath the obducted Samail Ophiolite; there is no robust evidence for an ∼110 Ma event or for SW‐dipping subduction beneath the Arabian plate. Combined with other constraints, our data suggest that the As Sifah unit was subducted at rates similar to other small continental HP bodies, followed by two stages of cooling and exhumation—likely associated with the dragging to mantle depths by a mafic root, an initial phase of rapid exhumation from mantle depths, and a lengthy residence (≤40 My) in the lower to middle crust.

Petrogenesis and tectonic significance of Paleoproterozoic metavolcanic rocks in the Khondalite Belt, North China Craton

As a typical continent–continent collision belt, the Khondalite Belt (KB) of the North China Craton (NCC) represents a Paleoproterozoic orogenic belt that underwent subduction and collision. Although many high-grade metamorphic rocks have been well studied, the low to medium grade metamorphic rocks well preserved in KB are less studied, which hinders a better understanding of the initiation of Paleoproterozoic subduction and collision processes. The Erdaowa Group is a representative of such a metamorphic rock series. Zircon U-Pb dating of the metavolcanic rocks in the Erdaowa Group yields the eruption ages of 2465 ± 24 Ma and 2350 ± 34 Ma, and the metamorphic ages of 1933 ± 32 Ma and 1844 ± 30 Ma. The geochemical results show that the metavolcanic rocks of the Erdaowa Group belong to tholeiitic basalts series. The enrichment of large ion lithophile elements (LILEs) such as Rb and Ba, and the depletion of high field strength elements (HFSEs) such as Ta, Nb and Ti indicate that the metavolcanic rocks have arc geochemical affinities and may have been formed at an active continental margin associated with subduction. Zircon Lu-Hf isotopic analysis shows that εHf(t) values are −0.15 – + 8.72, and the single-stage depleted mantle model age is 2448 – 2707 Ma, indicating that the primary magma originated from partial melting of the depleted lithospheric mantle and experienced slight crustal contamination. The results further prove that the low and medium grade rocks were directly involved in the subduction and collision between the Yinshan Block and the Ordos Block.

The Nico Pérez terrane (Uruguay) and its archean and paleoproterozoic inheritance

A U-Pb SHRIMP zircon geochronological study was carried out in the Nico Pérez Terrane in the central-eastern portion of Uruguay with the aim of constraining its geological evolution and its cratonic affinity. Nico Pérez Terrane is made up by a mosaic of tectonic blocks with diferente lithologies and sizes. This terrane is limited to the west by the Piedra Alta Terrane through the Sarandí del Yí shear zone, and to the east, is in tectonic contact through the Retamosa thrust with the Dom Feliciano Belt (Brasiliano orogenic cycle). Lithologically, the Nico Pérez Terrane is composed by medium to high grade metamorphic rock contained in three tectonic blocks (Pavas, Valentines and Rivera blocks) represented mainly by granitoids, as well as ortho and parametamorphic rocks, such as amphibolites, metapyroxenites, BIFs, schists, quartzites with fuchsite, among others. Archean inheritance, Rhyacian, Statherian, and Neoproterozoic magmatic ages are reported here. Similar ages, which show Archaean inheritance and Neoproterozoic imprint, reported in units of the Piedra Alta Terrane modify the idea that the Río de La Plata Craton only corresponds to a juvenile Paleoproterozoic tectonic unit. This, together with recently published geophysical information, supports the cratonic affinity of the Nico Pérez Terrane with the Río de La Plata Craton.

Remote sensing analysis of unnamed crater in Eastern Australia

Remote sensing (RS) can certainly provide deep insights about detecting the terrestrial structure of unknown origin. In this paper, we also detected impact crater of unknown origin in northeast Australia by RS techniques, specifically to enhance the credibility of scientific database on the possible impact craters in the continent of Australia. Following the RS procedures, a circular-shaped unnamed crater, hereafter the Winton crater, was detected with a diameter of approximately 130-km. Furthermore, the topographical parameter was obtained from RS data, which showed that the area, depth and volume of the crater are ~100-m2, ~130-m and ~99.8-m3, respectively. The geological data revealed that inside the crater, the outcrops are mainly consisted of sedimentary and low grade metamorphic rock, specifically included the mixed sediments and conglomerates, limestone and siltstone of the Craterous period. However, the exterior of the circular shaped in the southern part is consisted of unconsolidated deposits of the Tertiary period. The positive value of gravity anomaly for the major part of the crater is 3000 mGal and Bouguer gravity onshore grid has an anomaly of 900 mGal over the impact crater. It showed that the Winton crater could not be the due to any volcanic or karstic processes. On the other hand, a detailed field and petrology investigation should need to distinguish the origin of the crater of old and fossil travertine or an impact crater.

Application of BEMD in Extraction of Magnetic Anomaly Components Associated with Sn-W Polymetallic Mineralization in SE Yunnan, SW China

The tin (Sn)-tungsten (W) polymetallic ore concentrated district in SE Yunnan is distributed at the junction region of the Yangtze Block, the Cathaysian Block and the Indosinian Block, where there are several giant deposits of tin, tungsten, copper, silver, lead, zinc and indium closely associated with a large scale Late Cretaceous magmatism. Bi-dimensional empirical mode decomposition (BEMD) is used to extract aeromagnetic anomalous components at the survey scale of 1: 200 000 from the original aeromagnetic data of SE Yunnan. Four intrinsic mode functions (IMFs) and a residues component are obtained, which may reflect the geological structures and geological bodies at different spatial scales from high frequency to low frequency. The results are shown as follows: (1) Two different types of Precambrian basement in the study area were recognized: one is the Yangtze Block basement characterized by a strong positive magnetic anomaly, the other is the Cathaysian Block basement with a weak negative magnetic anomaly. The former consists of high grade metamorphic rocks including metamorphosed basic igneous rocks, while the latter consists of low grade metamorphosed sedimentary rocks. (2) The aeromagnetic anomalies associated with Sn-W polymetallic mineralization and related to granites in the study area illustrate a pattern of a skarnized alteration-mineralization zone with a positive ring magnetic anomaly enclosing a granitic intrusion with negative magnetic anomaly; (3) The ring positive magnetic anomaly zones enclosing the negative magnetic anomaly are defined as the Sn-W polymetallic ore-searching targets in the study area.

Crystallization Conditions and Petrogenetic Characterization of Metaluminous to Peraluminous Calc-Alkaline Orogenic Granitoids from Mineralogical Systematics: The Case of the Cambrian Magmatism from the Sierra de Guasayán (Argentina)

The Sierra de Guasayán (Eastern Sierras Pampeanas, Argentina) is formed by low to medium grade metamorphic rocks intruded by Cambrian metaluminous (La Soledad quartz-diorite), slightly peraluminous (Guasayán, El Escondido and El Martirizado granodiorite plutons), and strongly peraluminous (Alto Bello granodiorite) granitoids of the Pampean magmatic arc. Chemical compositions of amphibole, plagioclase, biotite, and titanite indicate that these granitoids were emplaced at low pressure (mostly &lt;3 kbar) and temperature (&lt;770 °C) under oxidizing conditions (QFM + 1 and QFM + 2), which are similar to the emplacement conditions reported for other granites of the Pampean magmatic arc. Mineral assemblages and whole-rock and mineral chemistry of the granitoids from the Sierra de Guasayán indicate an I-type affinity for the La Soledad quartz-diorite (amphibole, biotite, and titanite), S-type affinity for the Alto Bello granodiorite (biotite, muscovite, cordierite, and sillimanite), and a hybrid nature for the main Guasayán and El Escondido plutons (biotite, monazite, and magnetite). This hybrid nature is supported by the presence of abundant mafic microgranular enclaves and rapakivi texture and by published zircon Hf-isotope data (εHfi ranging from −4.76 to −0.12). This suggests, in turn, the involvement of hybridization in the genesis of these granitoids, which seems to be a common mechanism operating in the Pampean magmatic arc.

Geochemical characteristics, U-Pb dating of hydrothermal titanite from the Xingfengshan tungsten deposit in Hunan Province and their geological significance

杏枫山钨矿床是湘中地区近年新发现的、产出于浅变质岩系中的矽卡岩型钨矿。本文通过电子探针与LA-ICP-MS等技术手段，对杏枫山矿区矽卡岩中的热液榍石进行了主、微量元素组成以及U-Pb年代学研究。研究表明，榍石具有以下特征：（1）较低的∑REE、TiO₂含量，极低的Th/U，以及较高的Al₂O₃和F含量，这些均为典型热液榍石的地球化学特征；（2）富W而亏损Mo，表明其形成的热液流体具有富W贫Mo的特征；（3）稀土配分模式为左倾的、轻稀土显著亏损的重稀土富集型，正Ce、Eu异常表明其形成环境较为还原。此外，榍石原位U-Pb定年结果显示该矿形成于215.2±2.7Ma，与邻近白马山钛铁矿花岗岩形成的时间一致，基于地质及年代学证据，我们认为该矿的形成与白马山印支期的岩浆热液活动有关。结合前人的研究结果，我们发现湘中地区钨成矿与印支期还原性花岗质岩浆岩密切相关，为湘中印支晚期区域性成矿作用的产物。;The Xingfengshan skarn-type tungsten deposit, hosted in low grade metamorphic rocks, was discovered in Xiangzhong region recently. In this paper, major and trace element compositions, U-Pb dating of the hydrothermal titanite from the Xingfengshan skarn were analyzed by EPMA and LA-ICP-MS. The results show that the titanite possesses relatively low ∑REE, TiO₂ contents and Th/U ratio, and high Al₂O₃ and F contents, i.e., it belongs to typical hydrothermal titanite. The enrichment of W and depletion of Mo in the titanite suggest the hydrothermal fluid was rich in W and poor in Mo. The chondrite-normalized REE pattern of the titanite displays HREE enriched with obvious Ce and Eu positive anomalies, suggesting it formed under reduced environment. In-situ U-Pb dating of the titanite yields the intercept age of 215.2±2.7Ma, indicating the skarn tungsten deposit formed synchronously with the adjacent Baimashan ilmenite-type granitoid. Based on geological and geochronological evidences, we conclude that the deposit was genetically related to Baimashan magmatic-hydrothermal event in Indosinian. Combined with previous research, it is revealed that tungsten mineralization in Xiangzhong region is closely associated with Indosinian reduced granitoid and is a part of Late Indosinian regional mineralization event that took placed in the study region.

Provenance, clastic composition and their impact on diagenesis: A case study of the Oligocene sandstone in the Xihu sag, East China Sea Basin

Sandstone composition, heavy mineral analysis, and bulk rock geochemistry analyses were used to investigate the provenance lithotypes and sediment transport pathways of the third, fourth, and fifth (E3h3, E3h4 and E3h5) members of the E3h Formation at the gas–reservoir scale in the Xihu sag, East China Sea Basin. In addition, the diagenetic features and porosity of the metamorphic-sourced and volcanic-sourced sandstones were investigated. The provenance lithotypes of the E3h3, E3h4, and E3h5 were mainly medium–low grade metamorphic rocks and felsic to intermediate volcanic rocks. The sediment transport pathways were from northeast to southwest and from east to west, indicating that the E3h3, E3h4 and E3h5 members had two sediment provenance, namely, the metamorphic rocks of the northern Hupijiao Rise and the volcanic rocks of the eastern Diaoyudao Rise. Then the sedimentary microfacies distribution of the E3h3, E3h4, and E3h5 members and its relationship with the dual provenance sedimentary system were determined by using the heavy mineral assemblages, ZTR index, grain sizes, lithic fragments and quartz content. The initial porosity values of the metamorphic-sourced medium-grained and fine-grained sandstones were 35.35% and 35.22%, respectively; while the initial porosity values of the volcanic-sourced medium-grained and fine-grained sandstones were 34.09% and 34.97%, respectively. The average porosity compaction loss percentages were mainly 80.2% and 86.91% for the metamorphic-sourced medium–grained and fine–grained sandstones, and 86.95% and 87.23% for the volcanic-sourced medium–grained and fine–grained sandstones. The contact types of mineral grains in the metamorphic-sourced sandstones were mainly punctual, linear and concave-convex contacts; while the contact types of mineral grains in the volcanic-sourced sandstones were mainly linear, concave-convex, and sutured contacts. The metamorphic-sourced sandstone mainly developed chlorite coats; while the volcanic-sourced sandstones mainly contained pore-filling chlorite. The average quartz cement contents were 0.83% and 0.61% for the metamorphic-sourced medium–grained and fine–grained sandstones, and 0.67% and 0.43% for the volcanic-sourced medium–grained and fine–grained sandstones. The illite, mixed-layer illite/smectite and calcite cement reduced the sandstone porosity by filling pores. The metamorphic–sourced sandstones mainly developed primary intergranular pores and intergranular dissolved pores; while the volcanic–sourced sandstones mainly developed intergranular dissolved pores, intragranular dissolved pores, and moldic pores. The metamorphic-sourced sandstones had a higher porosity than the volcanic-sourced sandstones with the same grain size, which the difference in their porosities reached 1–3%.

Synkinematic interplay between felsic dykes and host rock mylonitization: how magmatism assists the formation of ductile narrow shear zones in the Sierra Chica de Córdoba, Argentina

This study provides new insights about mechanisms of synkinematic interplay between dyking emplacement and ductile narrow shear zones formation in the Sierra Chica de Córdoba. A detailed analysis of structural, microstructural, geometric-scaling and geochronologic data is performed in order to demonstrate the important role that minor and meso-scale felsic dykes played in the formation of paired shear zones symmetrically from dyke contacts. This magmatism and associated deformation conspicuously overprint medium-to high grade metamorphic and igneous rocks of Pampean age. Internally, the shear zones are formed by mylonites with mineral paragenesis in the amphibolite facies and foliation planes totally concordant to dyke contacts. Low rheological contrast between igneous intrusions and host rocks, together with fast rates of magma channeling and emplacement, was responsible for a brittle mechanical regime at moderate to high temperatures (500–700 °C). Melt-filled shear fractures rapidly propagated while the host rock was being mylonitized at ductile conditions. Shear structures controlling dyke construction and mylonite formation adjust to a strike-slip dominated dextral transtensional regime. The predicted axes of finite maximum shortening (Z) and maximum extension (X) have NNE and NNW directions, respectively. Ar/Ar muscovite cooling ages determined for pegmatite dykes, yielded minimum crystallization ages between 451 ± 1 and 435 ± 3 My. These geochronological data are further supported by field structural relationships and the regional knowledge, pointing to a Famatinian age for magmatism and related narrow shear zones. This paper constitutes a new contribution reporting the occurrence of magmatism and strike-slip-dominated dextral transtension deformation in the inner foreland of the Famatinian accretionary orogen, developed in the Lower Paleozoic along the southwestern Gondwana margin.