Metasedimentary Research Articles

Geochemistry of low-grade metasedimentary rocks from the Biga Peninsula, NW Turkey: Implications for provenance and tectonic setting

. Simplified tectonic map of the Eastern Mediterranean region indicating the major geotectonic units and the bounding sutures (modified from .

Mesoproterozoic magmatism redefines the tectonics and paleogeography of the SW Yangtze Block, China

The geodynamic setting of the Yangtze Block and its paleogeography in the reconstructed Rodinia supercontinent during the late Mesoproterozoic is loosely constrained due to the limited geological information reported. In this contribution, we identify a new suite of amphibolites in the SW Yangtze with an 1175 ± 5 Ma emplacement age. Geochemical modeling demonstrates they were derived from low-degree (3.5%–6.5%) partial melting of spinel lherzolite mantle. Additionally, three suites of late Mesoproterozoic granites in the SW Yangtze Block are revisited by providing new zircon U-Pb-Hf-O isotopes and whole-rock geochemical data. The granites have been dated to emplace during 1060–1042 Ma and have Zr + Nb + Y + Ce and Tzr values mostly at 304–576 ppm and 831–886 ℃, respectively, similar to A-type granites. Zircon Hf-O isotopic data indicate two subgroups of granites, in which one group has suprachondritic εHf(t) values of + 2.4 to + 11.4 with δ18O values of 6.5‰ to 7.4‰ while the other group has εHf (t) values of −3.9 to +1.5 with δ18O values of 10.0‰–11.6‰. The first group of granites was mainly derived from partial melting of mafic tholeiitic rocks while the second group of granites was mainly formed by partial melting of metasedimentary rocks. Integrated with published work, we propose that an intra-continental rift basin was developed in the SW Yangtze Block during ∼ 1.2–1.04 Ga, arguing against a late Mesoproterozoic orogenic event in the SW Yangtze Block. Statistical analyses based on compiled zircon U-Pb-Hf isotopes from Laurentia, Yangzte, and Amazonia show that the Yangtze has an isotopic affinity with west Nuna. These continents likely remained united during the transition from Nuna/Columbia to Rodina along the western periphery of the Rodinia supercontinent.

Zircon U-Pb chronology and Lu-Hf isotopic characteristics of the Pre-Sinian Wumaqing Formation and its intrusions in Yakou area, Miyi County, Sichuan Province, and their geological significance

在扬子板块西缘断续分布有多套变质沉积岩系，厘定区内不同岩石单元的时代对于探讨扬子西缘的构造演化具有重要意义。本文对出露于扬子板块西缘米易垭口地区的五马箐组变沉积岩以及侵入其中的片麻状黑云二长花岗岩体进行了锆石LA-ICP-MS U-Pb定年和Lu-Hf同位素分析。结果表明:区内五马箐组的沉积时代介于1.19~1.01Ga，其地层归属为会理群而非康定群，位于五马箐组之下的“冷竹关组”并非变沉积地层而是一套片麻状黑云二长花岗岩体。五马箐组碎屑锆石年龄存在~1.56Ga、~2.50Ga两个主要峰值以及~1.43Ga、~1.68Ga、~1.87Ga、~2.32Ga、~2.68Ga五个次要峰值，推断其物质来源主要为扬子板块西南缘的撮科杂岩、隐伏的或还暂未发现的太古代基底岩石、同时期岩浆岩和早期变沉积岩的再循环。碎屑锆石的ε_Hf（t）值在~1.7Ga发生了显著变化，暗示扬子板块西缘的构造体制由陆壳汇聚向裂谷活动转变，且在~1.56Ga处于裂谷岩浆活动的峰期。花岗岩体正的ε_Hf（t）值反映出扬子板块西缘在~1.0Ga有一次新生陆壳生长事件，可能是格林威尔造山后的伸展塌陷构造背景下裂谷岩浆活动的产物。;There are several sets of metasedimentary rock series distributed intermittently in the western margin of Yangtze Block, it is of great significance to determine the ages of different rock units in the area for discussing the tectonic evolution. This study has conducted zircon LA-ICP-MS U-Pb dating and Lu-Hf isotopic analysis of the metasedimentary rocks from the Wumaqing Formation as well as its intrusive gneissose biotite adamellite in Yakou area, western margin of Yangtze Block. The results indicate that the sedimentary age of Wumaqing Formation in the area is between 1.19~1.01Ga, and the strata belongs to Huili Group rather than Kangding Group. The Lengzhuguan Formation under the Wumaqing Formation is not a metasedimentary stratum but a set of gneissose biotite adamellite. The detrital zircon dating of the Wumaqing Formation have two main peaks of~1.56Ga and~2.50Ga, five minor peaks of~1.43Ga,~1.68Ga,~1.87Ga,~2.32Ga and~2.68Ga, it is speculated that its sedimentary material sources are mainly from Cuoke complex, buried or undiscovered Archean basement, contemporaneous magmatic rocks and the recycled of early metasedimentary rocks in the southwestern margin of Yangtze Block. The ε_Hf(t) values of detrital zircons has changed significantly at~1.7Ga, suggesting that the tectonic regime in the western margin of Yangtze Block has shifted from continental crust convergence to rift activities, and the rift magmatism has peaked at~1.56Ga. The positive ε_Hf(t) values of granitic pluton implies that there was a new continental crust growth event at~1.0Ga in the western margin of Yangtze Block, possibly is the product of rift magmatism under the extensional collapse tectonic setting after the Grenvillian Orogeny.

Geochronology, Geochemistry, and Zircon Hf-Isotopic Systematics of the Middle Cambrian Igneous Suites and Metasedimentary Rocks in Western Tasmania and Central Victoria

Geochronology, Geochemistry, and Zircon Hf-Isotopic Systematics of the Middle Cambrian Igneous Suites and Metasedimentary Rocks in Western Tasmania and Central Victoria

НОВЫЙ ВЗГЛЯД НА СТРАТИГРАФИЮ СЕЛЕМДЖИНСКОГО И ТОКУРСКОГО ТЕРРЕЙНОВ МОНГОЛО-ОХОТСКОГО ПОЯСА: РЕЗУЛЬТАТЫ U-Pb, Lu-Hf, Sm-Nd ИЗОТОПНЫХ ИССЛЕДОВАНИЙ

Results of U-Pb and Lu-Hf isotopic studies on detrital zircons from metasedimentary rocks in the eastern part of the Selemdzha and Tokur terranes of the Mongol-Okhotsk belt and Sm-Nd isotopic data for whole-rocks are in complete contradiction with existing ideas about the stratigraphy of the region under consideration and indicate the need to revise the traditional principles of mapping within the belt. Two types of deposits have been established, differing in Sm-Nd isotopic characteristics of whole-rocks and Lu-Hf isotopic composition of detrital zircons. Deposits of different types are involved in the formation of the two opposing accretion systems: in front of the Siberian Craton margin (type I) and the Amur superterrane (type II). Among type I deposits, discrete groups are distinguished that differ in the lower age limit: 553–498 Ma, Late Ediacaran-Cambrian; ~373 Ma, Upper Devonian; 333–327 Ma, Upper Mississippian; ~304 Ma, Pennsylvanian. The deposits of tectonic-stratigraphic units show a general pattern of younging from north to south. Thus, we observe a structure typical of accretionary wedge-shaped terranes with a rear part in the north and a frontal part in the south. Tectonic-stratigraphic units formed by type I deposits and previously considered part of the Selemdzha and Tokur terranes, apparently, are components of the Galam terrane of the accretionary wedge.

The Chemical and B-Isotope Composition of Tourmaline from Intra-Granitic Pegmatites in the Las Chacras-Potrerillos Batholith, Argentina

ABSTRACT The Devonian Las Chacras-Potrerillos batholith comprises six nested monzonitic to granitic intrusions with metaluminous to weakly peraluminous composition and a Sr-Nd isotopic signature indicating a dominantly juvenile mantle-derived source. The chemically most evolved units in the southern batholith contain a large number of intra-granitic, pod-shaped tourmaline-bearing pegmatites. This study uses in situ chemical and boron isotopic analyses of tourmaline from nine of these pegmatites to discuss their relationship to the respective host intrusions and the implications of their B-isotope composition for the source and evolution of the magmas. The tourmalines reveal a diversity in element composition (e.g., FeO, MgO, TiO2, CaO, MnO, F) which distinguishes individual pegmatites from one another. However, all have a narrow δ11B range of –13.7 to –10.5‰ (n = 100) which indicates a relatively uniform magmatic system and similar temperature conditions during tourmaline crystallization. The average δ11B value of –11.7‰ is typical for S-type granites and is within the range reported for peraluminous granites, pegmatites, and metamorphic units of the Ordovician basement into which the Las Chacras-Potrerillos batholith intruded. The B-isotope evidence argues for a crustal boron source like that of the Ordovician basement, in contrast to the metaluminous to weakly peraluminous composition and juvenile initial Sr and Nd isotope ratios of the Las Chacras-Potrerillos batholith magmas. We propose that the boron was not derived from the magma source region but was incorporated from dehydration melting of clastic metasedimentary rocks higher up in the crustal column.

PETRO-MECHANICAL STUDIES OF THE STRATIGRAPHIC UNITS OF THE TARKWAIAN SUPERGROUP IN TARKWA – IMPLICATIONS FOR THE STRUCTURAL AND MECHANICAL COMPETENCE OF ROCKS

Main purpose of this paper is to determine the mineralogical composition and mechanical properties of the Tarkwaian Supergroup and compare them. This project will help determine whether the rock units of the Tarkwaian Supergroup are competent based on their mineral contents and strength of the rocks to be quarried and used as aggregates for construction and also determine the structural features that were formed in the various rock units due to the deformation that took place. Values obtained from the Unconfined Compressive Strength (UCS) test were (21.50-77.10) N/mm2 with the Kawere conglomerate having the lowest, and Tarkwa Phyllite having the highest strength. The Huni sandstone, Banket Quartzite and Kawere conglomerate were classified as weak rocks, but the Tarkwa Phyllite was classified as a medium rock, according to Attewell and Farmer’s (1976) classification. The Huni sandstone, Banket Quartzite and Kawere conglomerate are made up of grains of varying degrees of roundness and maturity. The Huni sandstone, Banket Quartzite, and Tarkwa Phyllite exhibit foliation. Micro-folding is also observed in the Kawere conglomerate. Petrographic studies of the rocks classify them as metasedimentary rocks weathering. Based on the weak strengths of the various rock units, they are not suitable to be used as aggregates for construction but can be used for other purposes such as construction sand and dimension stones.

Recognition of broad thermal anomaly around the median tectonic line in central Kii peninsula, southwest Japan: Possible heat sources

AbstractApplication of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low‐grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease in temperature from ~390 °C close to the northern boundary, a major continental shear zone—the median tectonic line (MTL)—to ~270 °C and ~7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip‐on fault boundaries between different geological units or folding. Meso‐ and microstructural observations combined with strain analysis using detrital grains in meta‐mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and are not due to localized preferential exhumation along with the MTL. The observations also require a heat source along with the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons, fluid infiltration is our preferred model.

The role of phyllosilicate partial melting in segregating tungsten and tin deposits in W-Sn metallogenic provinces

AbstractMost tungsten (W) and tin (Sn) deposits are associated with highly evolved granites derived from the anatexis of metasedimentary rocks. They are commonly separated in both space and time, and in the rare cases where the W and Sn mineralization are part of a single deposit, the two metals are temporally separate. The factors controlling this behavior, however, are not well understood. Our compilation of whole-rock geochemical data for W- and Sn-related granites in major W-Sn metallogenic belts shows that the Sn-related granites are generally the products of higher-temperature partial melting (~800 °C) than the W-related granites (~750 °C). Thermodynamic modeling of partial melting and metal partitioning shows that W is incorporated into the magma formed during low-temperature muscovite-dehydration melting, whereas most of the Sn is released into the magma at a higher temperature during biotite-dehydration melting; the Sn of the magma may be increased significantly if melt is extracted prior to biotite melting. At the same degree of partial melting, the concentrations of the two metals in the partial melt are controlled by their concentration in the protolith. Thus, the nature of the protolith and the melting temperature and subsequent evolution of the magma all influence the metallogenic potential of a magma and, in combination, helped control the spatial and temporal segregation of W and Sn deposits in all major W-Sn metallogenic belts.

Detrital Zircon U-Pb Geochronology and Petrography Analysis of the Haenam Basin

The detrital zircons of the Cretaceous Haenam basin were examined to determine how they responded to the subduction of the oceanic plate to the eastern Asian continental margin during the Cretaceous. A sinistral strike-slip movement caused by the oblique subduction of the Paleo-Pacific plate created these two nonmarine subbasins in the northern marginal Okcheon Belt of the Korean Peninsula. In the late Cretaceous, due to the orthogonal subduction of the oceanic plate, sediments were deposited in terrestrial environments with associated volcanism. A total of 247 ages obtained from 300 zircon grains reveal that the maximum depositional ages of the Haenam basin is ca. 79.5 0.17 Ma, respectively. The detrital zircon age spectra indicate that their basin fills were mainly derived from the adjacent basement rocks comprising Paleoproterozoic metamorphic rocks and Jurassic granitoids with a minor supply from the Paleozoic metasedimentary rocks in the western Gyeonggi Massif and Okcheon Metamorphic Belt.

Diagenetic and epigenetic origins for Cu‐Ag mineralization in the Khoemacau Zone 5 deposit, Kalahari Copperbelt, northwestern Botswana

AbstractIn the Kalahari Copperbelt, the mechanism of regional scale Cu‐Ag mineralization has remained intensely debated between early diagenesis and a single pass during a regional deformation event. At the Zone 5 Cu‐Ag deposit in northwestern Botswana, the orebody is hosted preferentially by chemically reduced metasedimentary rocks that overly oxidized, hematite‐bearing arkosic sandstone. An early diagenetic mineralizing event has been identified which is characterized by fine‐grained stratiform pyrite, including recrystallized framboidal pyrite, intergrown with diagenetic mineral assemblages in the host‐rock. Diagenetic pyrite is in textural equilibrium with chalcopyrite, sphalerite, galena, and (Fe‐Co‐Ni) sulfarsenide. These minerals were subsequently overprinted by a more intense, multi‐stage, structurally‐controlled hydrothermal Cu‐Ag mineralization event related to the Damaran orogeny (~600–480 Ma). The hydrothermal Cu‐Ag mineralization was deposited from hot (~236–265°C), high salinity (19–24.6 wt% NaCl equiv.) hydrothermal ore fluids. Petrographic results reveal an apparent overlap in trace metal associations (Cu, Fe, As, Zn, Pb, Ni, Co) between the two mineralizing events, which can be explained by remobilization of precursor sulfides. The major Ag‐carriers in the ore are chalcocite, covellite, and bornite. The δ34S values of diagenetic pyrite range from −35.8 to +11.4‰, whereas those of hydrothermal epigenetic sulfides, including pyrite, range from −28.0 to +3.0‰. We propose that the hydrothermal sulfides had acquired some bacterially‐reduced sulfur from earlier‐formed minerals. The δ18O and δ13C values of quartz and calcite associated with the hydrothermal mineralization are typical of Neoproterozoic sediment‐hosted Cu‐Ag deposits. However, the δ18O isotopic values of the calcite gangue are anomalously depleted, which is likely due to recrystallization under metamorphic conditions. Our studies at Zone 5 indicate that the Zone 5 Cu‐Ag deposit is the result of a multi‐stage mineralization history that includes both diagenetic and epigenetic events (punctuated by >400 m.y.) facilitated by a strong litho‐structural control.

Crustal Melting Evidence in Migmatites of Higher Himalayan Crystallines (HHC) along Bhagirathi, Dhauliganga Valleys, and Sikkim Himalaya, India

Abstract An attempt has been made to evaluate crustal melting evidence through textural studies within a narrow zone of migmatite present in NW Himalaya along Bhagirathi and Dhauliganga valleys and two zones close to MCT and throughout within in the Sikkim Himalayas, NE Himalaya. It appears that partial melting was initiated by muscovite dehydration melting with a positive volume change driving melt segregation and discontinuous crystallization of peritectic biotite in the leucosome. Further, during retrogression due to cooling, a certain amount of melt was consumed. The occurrence of isolated pseudomorphosed melt pockets and lack of euhedral magmatic flow textured feldspar further indicates that the melt fraction was low for the alignment of crystals. The processes of melt segregation and migration could have been limited. The migmatite leucosomes and a small volume of in situ tourmaline-bearing leucogranite along extensional crenulation cleavages and melt pods indicate water-saturated melting of pelitic metasedimentary rocks. The formation of migmatites happened at around 46 Ma, corresponding to a peak metamorphic event due to collisional tectonics of the Himalayan orogeny. The presence of feeder dikes for main tourmaline-bearing leucogranite indicates that the source for the main body could be migmatite which is also supported by the similarity in REE patterns of the main body and in situ tourmaline-bearing leucogranite.

Formation of the Late Cretaceous-Paleogene Sn-rich granites in the Tengchong block: The magma origin and differentiation conditions

腾冲地块锡成矿作用主要与晚白垩世-古近纪岩浆活动相关，但仅矿区花岗岩体具有较高的锡含量（平均25×10^-6），区域上同时代的非成矿花岗岩体并未发生锡的富集。本文通过搜集分析现有研究数据，总结了富锡成矿岩体和区域非成矿岩体在岩浆源区、演化条件和结晶分异程度的异同：区域非成矿岩体锆石ε_Hf（t）值（-15.1~+3.39）表明，自东北向西南幔源物质加入有升高的趋势，但区域玄武岩中低含量的Sn （1.61×10^-6）表明幔源物质混入不利于岩浆中锡的富集。晚白垩世部分非成矿岩体与成矿岩体具有相同ε_Hf（t）（-9.7）值，表明其皆源于古老地壳物质的部分熔融，但岩体均表现为准铝质-弱过铝质特征，且锆石Hf同位素（t_DM2=1724Ma）和全岩Nd同位素（t_DM2=1836Ma）二阶段模式年龄基本一致，因此其源区可能并非富锡的高黎贡山群变质沉积岩，而可能是其中未经风化的变质花岗岩。根据腾冲地块地层厚度（28km）和莫霍面深度（47~35km）推断岩浆源区至少位于地下30km （8.4kbar），由于仅靠地温梯度（25℃/km）无法达到初始熔融温度（>1066℃），源区部分熔融过程很可能受地幔热的影响。根据Fe₂O₃/FeO比值，非成矿岩体（0.59）与成矿岩体（0.48）均具有较低的氧逸度，属钛铁矿系列，但成矿岩体的结晶分异程度明显高于非成矿岩体，且成矿岩体富含挥发分，高含量的挥发分降低了岩浆固结温度（650~550℃），延长了结晶分异时间，促进了锡在晚期岩浆中的富集。因此腾冲地块富锡花岗岩主要是普通岩浆在低氧逸度环境下发生高度结晶分异的结果。;Tin metallogenesis in Tengchong block is associated with the Late Cretaceous and Paleocene magmatism. However, only the ore-forming granitoids are characterized by high Sn content (25×10^-6 on average). The regional contemporary unmineralized granitoids hardly show Sn enrichment. By collecting and analyzing available research material, the differences between the Sn-rich ore-forming rocks and regional unmineralized rocks are summarized in terms of their magma origins, evolution conditions and differentiation degrees. Zircon ε_Hf(t) values (-15.1~+3.39) of the regional unmineralized rocks indicate that the magmas in this area are added with increasing mantle material from northeast to southwest, while the addition of the mantel material is disadvantageous to tin enrichment in the mixing magma according to the Sn content (1.61×10^-6) of the regional basalt. Part of the regional unmineralized granitoids formed in Late Cretaceous have the same ε_Hf(t) values of -9.7 with the Sn-rich ore-forming rocks, suggesting both of them were derived from partial melting of ancient crustal material. Considering these rocks are all metaluminous or weak peraluminous, and the crustal model age (t_DM2) calculated from zircon Hf isotopes (1724Ma) are nearly identical to that from the whole-rock Nd isotopes (1836Ma), the magma may not be derived from the Sn-rich Gaoligongshan Group consisting of metasedimentary rocks, but probably the inner metamorphic granitoids. The metamorphic granitoids are at least 30km underground (corresponding to 8.4kbar) based on the regional strata thickness (28km) and the Moho depth (47~35km). The initial melting temperature (1066℃) is unable to be reached according to geothermal gradient (25℃/km), therefore, the partial melting process is likely influenced by mantle heat. According to Fe₂O₃/FeO values, both the unmineralized granitoids (0.59) and the tin mineralized granitoids (0.48) have low oxygen fugacity, and belong to ilmenite series. While the fractionation of Sn-rich granitoids is much higher than that of unmineralized granitoids, moreover, the Sn-rich granitoids contain abundant volatiles, which decrease the magma consolidation temperature (650~550℃), prolong magma differentiation process, and promote Sn gathering in the late magma. In conclusion, the Sn-rich granitoids in the Tengchong block is mainly resulted from the high fractionation of normal magma under a low oxygen fugacity.

Geochemistry and U–Pb zircon age of the Paleoproterozoic metasedimentary rocks from the Bidou I, Nyong Series, Cameroon: Implications for provenance and tectonic setting

Geochemistry and U–Pb zircon age of the Paleoproterozoic metasedimentary rocks from the Bidou I, Nyong Series, Cameroon: Implications for provenance and tectonic setting

Mineral Composition of Rare Element Enriched Rock Bodies from the Bükk Mts., NE Hungary

A rare earth (REE) and further rare element (Zr, Nb, Th, Ta) enrichment was found recently within the CriticEl project in 2016. Due to continuing studies, now we know about several enriched rock bodies from the SE and NE part of the Bükk Mts. The enrichment was caused by metasomatic processes, during which fluids penetrated through the rock bodies, causing chemical and mineralogical changes. The enriched rock bodies are Triassic metavolcanics and siliciclastic metasedimentary rocks. The protoliths consist of quartz, feldspars (albite and potassic feldspars), phyllosilicates (various micas and chlorites) and calcite, and in minor quantities Fe- and Ti-oxides, fluorapatite and titanite. The characteristic metasomatic minerals are monazite-(Ce), zircon and Nb-bearing Tioxide. Based on our data, micas and potassic feldspars were generated at least partly during the rare element mineralization process, while titanite, chlorite and albite were consumed. Apatite and Ti-oxides were decomposed and formed. The original mineral composition had a strong influence on the rate and mode of alteration.

Metamorphic evolution of graphite in the Paleoproterozoic Savo Schist Belt (SSB), Central Finland: Constraints from geothermetric modeling

Flake graphite deposits of the Savo Schist Belt (SSB), Central Finland are mainly hosted by quartz-mica schist, hornblende biotite gneiss, and to a minor extent in amphibolite. These rocks represent high-grade metamorphic conditions of the SSB. Raman spectroscopy measurements on graphite flakes provide quantitative information on the degree of graphitization, which can be correlated to the grade of metamorphism. The metamorphic conditions of the graphite-bearing metapelites have been determined by P-T pseudosection modelling and three types of geothermometers: garnet-biotite (GB), Ti-in biotite and Raman spectroscopy of carbonaceous material (RSCM) geothermometry. The calculated P-T pseudosections indicate peak P-T conditions of ca. 5–6 kbar and 650 °C for two garnet-bearing hornblende biotite gneiss samples, and slightly higher conditions of ca. 5–6 kbar and 700 °C for a quartz mica schist sample. The GB thermobarometer yielded a temperature and pressure range of 600–725 °C and 5.2–8.0 kbar. The Ti-in-biotite applied to the same samples produced temperature ranging between 570 and 735 °C, and the Raman (RSCM) geothermometers indicated temperatures between 550 and 670 °C. For the garnet-bearing hornblende biotite gneiss samples there is no significant (<10–50 °C) difference between temperatures determined using the P-T pseudosections and mean temperatures calculated using RSCM, GB and Ti-in-biotite methods. Although the peak temperature in some highly crystalline graphite flakes of studied samples calculated by (RSCM) geothermometer yield temperatures of 641 °C with the Beyssac et al. 2002 method and 737 °C with the Rahl et al. (2005) method, which are the highest temperature for this calibration. Raman spectroscopy is now part of a wide selection of tools that can be used to identify the ordering of the graphite crystallinity, which is including the number of layers, quality of layers, doping level and confinement in graphene structures. The results of this work show that Raman spectroscopy is a simple and fast way to determine the peak metamorphic temperature of carbonaceous material CM in amphibolite facies metasedimentary rocks.

Multiple sediment incorporation events in a continental magmatic arc: Insight from the metasedimentary rocks of the northern North Cascades, Washington (USA)

Abstract The rheology and composition of arc crust and the overall evolution of continental magmatic arcs can be affected by sediment incorporation events. The exhumed Cretaceous–Eocene North Cascades arc exposes abundant metasedimentary rocks that were incorporated into the arc during multiple events. This study uses field relationships, detrital zircon geochronology, bulk rock geochemistry, geothermometry, and quartz-in-garnet geobarometry to distinguish approximate contacts and emplacement depths for different metasedimentary units to better understand their protolith incorporation history and impact on the arc. The Skagit Gneiss Complex is one of the main deep crustal units of the North Cascades arc. It includes metasedimentary rocks with distinct detrital zircon signatures: Proterozoic–Cretaceous (Group 1) or Triassic–Cretaceous (Group 2) zircon populations. Both metasedimentary groups achieved near-peak metamorphic conditions of 640–800 °C and 5.5–7.9 kbar; several Group 2 samples reveal the higher pressures. A third group of metasedimentary rocks, which was previously interpreted as metamorphosed equivalents of backarc sediments (Group 3), exhibited unimodal Triassic or bimodal Late Jurassic–Early Cretaceous detrital zircon signatures and achieved near-peak conditions of 570–700 °C and 8.7–10.5 kbar. The combined field and analytical data indicate that protoliths of Group 1 and Group 2 metasedimentary rocks were successively deposited in a forearc basin and underthrusted into the arc as a relatively coherent body. Group 3 backarc sediments were incorporated into the arc along a transpressional step-over zone. The incorporation of both forearc and backarc sediments was likely facilitated by arc magmatism that weakened arc crust in combination with regional transpression.

Regional geology and tectonic framework of the Southern Indian domain, Trans-Hudson orogen, Manitoba

The collaborative federal–provincial Southern Indian Lake project in north-central Manitoba covered an area of more than 3500 km2 of the Trans-Hudson orogen. Regional-scale geological mapping, sampling, and lithogeochemical, isotopic, and geochronological studies resulted in the identification of distinct assemblages of supracrustal rocks and varied episodes of plutonism. A granodiorite gneiss dated at ca. 2520 Ma is interpreted to represent the basement of the Southern Indian domain and is considered a separate crustal domain, named the Partridge Breast block. The Churchill River assemblage is composed of juvenile pillow basalt with intervening clastic sedimentary rocks, possibly a reflection of plume magmatism related to initial rifting of the Hearne craton margin. The Pukatawakan Bay assemblage consists mainly of massive to pillowed, juvenile metabasaltic rocks and associated basinal metasedimentary rocks. The Partridge Breast Lake assemblage is dominated by continental-arc volcanic and volcaniclastic rocks associated with basinal metasedimentary rocks. The Strawberry Island assemblage, consisting of arenite and polymictic conglomerate, is interpreted to have been deposited in a foreland-basin basin or intra-orogen pull-apart basin environment. The Whyme Bay assemblage is characterized by fluvial-alluvial orogenic sediments and is temporally linked to the Sickle Group rocks in the Lynn Lake greenstone belt. Granitoid rocks, dominantly monzogranite and granodiorite, range in age from ca. 1890 to 1830 Ma and occur throughout the Southern Indian domain, and intermediate and mafic intrusions of similar ages are also present. In this paper, we integrate these new data into a tectonic framework for the Southern Indian domain of the Trans-Hudson orogen in Manitoba.

Age and structural constraints on the tectonic evolution of the Paleoproterozoic Saimaa orocline in Fennoscandia

The Paleoproterozoic oroclines of Fennoscandia played a vital role in the making of continental equidimensional crust during the Svecofennian orogeny. We propose that the 1.90–1.88 Ga arc rocks in south-central Finland originally formed a single linear arc system that is now represented by the coupled Bothnian oroclines and their southern continuation, the Saimaa orocline, as defined in this study. A regional large-scale approach with robust geochronological age determinations obtained from metasedimentary rocks and granitoids, and field-based structural studies are used to produce a coherent view on the tectonic evolution of the Saimaa orocline.The pre-orocline tectonic evolution started with a NE-SW collision between the Karelia continent (lower plate) and Proto-Svecofennia (upper plate) at ca. 1.91 Ga (D1). This was followed by subduction reversal and arc magmatism at ca. 1.90–1.88 Ga. Collision or flat subduction at 1.885 Ga caused a widespread collisional event (D2). Our results indicate that the presently E-W oriented D2 structures are not, as is widely proposed, products of N-S contraction but instead originated as a N-S to NNW-SSE oriented belt that was subject to E-W contraction. Dorocline, recorded as the first deformation stage in post-D2 sediments, folded the pre-existing structures into their E-W and ENE-WSW orientation. Following orocline formation, the main shortening direction switched back to the NE-SW (D3) resulting in type-1 and type-2 refolded folds, long wavelength regional folds and bending of the hinge zone of the Saimaa orocline. D4-D5 deformation stages are localized intra-continental tectonic events at 1.83–1.79 Ga. The formation of the Saimaa orocline is bracketed at 1875–1865 Ma and formed by margin-parallel shortening. The Bothnian and Saimaa oroclines form continuous buckles, but whereas the Bothnian coupled oroclines are crustal-scale parallel folds due to more rigid crustal core, Keitele microcontinent, the Saimaa orocline is a crustal-scale isoclinal similar fold.

Tectonic setting of the South Liaohe Group in the Jiao‐Liao‐Ji Belt, North China Craton: Geochemical and geochronological constraints from metasedimentary rocks

The metasedimentary rocks of the South Liaohe Group are widely distributed along the Jiao‐Liao‐Ji Belt (JLJB) in eastern Liaoning Province. It is important to understanding tectonic setting and evolution of the JLJB by determining its sediment sources and properties of the provenances, sedimentary facies, metamorphic ages, and facies. After the detailed field geological surveys and comprehensive laboratory observation, many high‐grade metapelitic rocks are identified in the South Liaohe Group. The main rock type is sillimanite‐garnet‐cordierite‐biotite plagiogneiss. The zircon U–Pb geochronology of three samples shows that these high‐grade meta‐pelitic rocks are mainly derived from the Liaoji Granites and the Archean basement. The protolith formation ages are between 2.00 and 1.93 Ga. Subsequently, these protolithes underwent high‐pressure metamorphism resulted from collisional orogeny, with peak metamorphism at ca. 1.88 Ga. Then, at 1.88–1.84 Ga, these rocks experienced the retrograde metamorphism and exhumed to the surface during crustal uplift. Geochemical data show that the protoliths are alumina‐rich claystones and graywackes with low maturity, implying near‐source sedimentation. The chemical index of alteration (CIA) values of these samples range from 60 to 80, indicating that the protoliths underwent moderate weathering. The sediment source was mainly felsic components of the upper crust, possibly with the addition of ancient sediments. Various tectonic environment discrimination diagrams combined with previous results on metamorphism and isotope chronology of these metasedimentary rocks in the JLJB indicate that the rocks in the study area originated from an active continental margin with an evolving island arc.