Neoproterozoic Metasedimentary Rocks Research Articles

Crustal anatexis recorded by zircon grains from early Paleozoic granitic rocks in Southeast China

Episodic granitic magmatism occurred in Southeast China from the Paleoproterozoic to the Late Cretaceous, and some of the early Paleozoic granitic rocks in the region are inferred to have formed through crustal anatexis. However, the complex processes associated with upper amphibolite-facies metamorphism and anatexis remain unclear. We investigate the early Paleozoic anatexis by analyzing zircon grains from granitic rocks in the northern Wuyi Mountains of southern Jiangxi Province. By combining mineral assemblage, field relationship, and thermodynamic modeling data with observations of zircon internal structures, we infer that anatexis occurred via partial melting of Neoproterozoic metasedimentary rocks, immediately following amphibolite-facies regional metamorphism at ca. 442 Ma (recorded by rare metamorphic zircon overgrowths around inherited cores). We identify three types of zircon grains/domains in the analyzed samples, which are related to anatectic melt migration, extraction, and eventual crystallization. Large variations in zircon Hf isotopic compositions and homogenization trends indicate that melting was heterogeneous, and homogenization of the anatectic melt occurred later. Petrogenesis of the granitic rocks, including metamorphism, disequilibrium melting, and melt accumulation, occurred over a short period from ca. 447 ± 8 Ma to 437 ± 3 Ma. We demonstrate a strong relationship between Neoproterozoic metasediments and early Paleozoic granitic rocks in Southeast China and show that homogeneous granitic plutons can form through the ‘rehomogenization’ of continental crust.

Provenance and tectonic setting of the Neoproterozoic meta-sedimentary rocks at southeastern Tibetan Plateau: Implications for the tectonic affinity of Yidun terrane

The Yidun terrane is located between the Qiangtang terrane and Songpan-Ganzi Fold Belt and its tectonic affinity with other adjacent terrane is of significance to understand the tectonic evolution of southeastern Tibetan Plateau. In this study, we present integrated bulk geochemical data, detrital zircon U-Pb ages and Hf isotopes of meta-sedimentary rocks from the Qiasi Group, which represents the oldest Precambrian basement of the Yidun terrane, to constrain the depositional age and provenance of the meta-sedimentary rocks as well as the tectonic affinity of the terrane. Zircon U-Pb dating results show that detrital zircons from three samples have almost identical age spectra with two major age clusters of 800–780 Ma and 850–820 Ma at age peaks of ca. 790 Ma and ca. 827 Ma, and one minor age cluster of 910–860 Ma. The youngest group of detrital zircons reveals that the maximum depositional age of the Qiasi Group was 779–777 Ma. Low SiO2/Al2O3 (2.9–4.1), CIA (59–47), and PIA (63–47) values indicate that the sediments could be derived mainly from the low mature sources that had undergone weak to mildly moderate degree of chemical weathering. Bulk geochemical and zircon U-Pb-Hf isotopic data suggest that the provenance was composed mainly of Neoproterozoic intermediate to felsic igneous rocks. In combination with the published geological data and detrital zircon U-Pb age spectra on the western Yangtze Block, and the Kunlun-Qilian-Qaidam Block, we propose that the Neoproterozoic basement of the Yidun terrane has a close tectonic affinity with the former one.

Mineralogical and isotope-geochemical (δ13C, δ34S and Pb-Pb) characteristics of the Krasniy gold mine (Baikal-Patom Highlands): Constraining ore-forming mechanisms and the model for Sukhoi Log-type deposits

Orogenic gold deposits localized in carbonaceous sedimentary (black-shale) complexes are major contributors to worldwide balance of gold extracted from the crust, though there is a limited consensus about the role of host sedimentary rocks, involvement of external (i.e., magma-derived) fluids and brittle deformations into ore remobilization and transport. This paper presents the original results of mineralogical, petrographic and isotope-geochemical studies of rocks and ores from the large Krasniy gold deposit (Baikal-Patom plateau, southern Siberian craton) given in comparison with the thoroughly studied giant Sukhoi Log deposit, with both deposits hosted by the Neoproterozoic carbonaceous metasedimentary rocks. The multi-stage ore process occurred discretely at 200–400 °C (greenschist-facies metamorphism) and included (1) low-P-T diagenetic formation of globular pyrite, (2) catagenetic (<200 °C, 1–2 kbar) formation of small euhedral pyrite, (3) high-temperature and high-pressure (300–380 °C, 5–6 kbar) crystallization of arsenopyrite and pyrrhotite related to metamorphic transformation, and (4) formation of the major ore assemblage (arsenopyrite, pyrrhotite, pyrite meta-aggregates, native gold, galena, sphalerite, fahl ores) during almost isothermal decompression (270–320 °C, 0.5 kbar). Gold is detected as nanoparticles unevenly distributed at the surface of the early globular pyrite (up to 0.4 ppm) and occurs as inclusions in later metamorphic pyrite. Relatively narrow ranges of sulfur (δ34S within −4.7 – −6.9‰) and lead isotopic composition (206Pb/204Pb = 18.33–18.65, 207Pb/204Pb = 15.65–15.68 and 208Pb/204Pb = 38.13–38.82) in sulfides from the ores was continuously inherited from earlier generation of sulfides to the later (associate with native gold) and is similar to the values obtained for sulfides from the host Neoproterozoic metasedimentary rocks of the Aunakit formation. The compositional inheritance together with geochemical evidence of no additional contribution of К, Na, Mg and Si into the ores and no correlation between gold and organic carbon content highlight the absence of the external metasomatic overprint of metal-bearing fluids. Instead, both the above facts and high Au content (up to 1 ppm) in bitumens derived from the host metasediments indicate a key role of the host carbonaceous substrate in the supply of metals through release and local transport of internally derived diagenetic/catagenetic carbon-rich fluids. Overall, the obtained results for the Krasniy deposit led to distinguishing the three key stages of the gold deposit formation, including 1) sedimentation of gold-specialized sediments in the spreading of the back-arc basin, where gold was associated with the scattered diagenetic pyrite mineralization (610–590 Ma), 2) catagenetic redistribution of sedimentary pore fluids from preconditioned sedimentary rocks into the vaulted parts of gentle anticlines with the formation of abnormally high concentrations of gold (~570 Ma), and 3) metamorphic-metasomatic redistribution of gold as a result of decarbonization of catagenic fluid with the formation of economically significant ore bodies (450–420 Ma). The compilation of the available geochemical and isotopic data allows to suspect this model as a general for the other Bodaibo region deposits, so that it can be used further for prediction and exploration purposes.

Genesis of early cretaceous leucogranites in the Central Sanandaj–Sirjan zone, Iran: Reworking of Neoproterozoic metasedimentary rocks in an active continental margin

New U–Pb ages and geochemical and isotopic data from leucogranites within the central Sanandaj–Sirjan zone (SSZ) of the Zagros orogen in Iran enable an evaluation of a migrating magmatic system in response to subduction of Neo-Tethyan lithosphere. The Esfajerd leucogranites, from the Golpaygan region of the Muteh–Golpaygan metamorphic complex, are composed mainly of tourmaline and two-mica leucogranites and are characterized by strongly peraluminous compositions. SIMS U–Pb zircon ages indicate that these rocks were emplaced over a 7 Myr period between 138 and 131 Ma. In multi-element variation diagrams, all samples display pronounced negative Eu anomalies, enrichment in light rare-earth elements and large-ion lithophile elements, and depletion in high-field-strength elements, Sr, and Ba. The tourmaline and two-mica leucogranites record positive zircon εHf values of 1.7–5.6 and 2.3–8.9, respectively, suggesting a predominantly Neoproterozoic crustal source. Zircon Hf–O isotopic compositions, together with the whole-rock geochemical data, show that both leucogranite types were produced by muscovite dehydration melting of metasedimentary sources under fluid-absent conditions, followed by fractional crystallization of the parental melt. The relatively high δ18O values (8.68–10.32‰) of the tourmaline leucogranites are consistent with their derivation from a psammite-dominated source, such as metagraywackes, whereas the wider range and higher maximum of the δ18O values (6.99–12.32‰) of the two-mica leucogranites correspond to mixed pelitic–psammitic sources, with little or no input of mantle-derived material. We suggest that the leucogranites were formed by melting of metasedimentary rocks during a stage of flat-slab subduction initiated by plate reorganization.

From Breakup of Nuna to Assembly of Rodinia: A Link Between the Chinese Central Tianshan Block and Fennoscandia

AbstractThe transition from breakup of Nuna (or Columbia, 2.0–1.6 Ga) to assembly of Rodinia (1.0–0.9 Ga) is investigated by means of U‐Pb and Lu‐Hf data of detrital zircons from three Neoproterozoic metasedimentary rocks in the Central Tianshan Block (CTB), NW China. These data yield six age peaks around 1.0, 1.13, 1.34, 1.4–1.6, 1.75, and 2.6 Ga. Few zircons are detected between 2.0 and 2.5 Ga. The Paleoproterozoic to Neoproterozoic detrital zircons have Hf isotopic compositions (−22.1 to +13.0) similar to those of coeval magmatic rocks in the CTB, indicating a proximal provenance. These results, together with the geological evidence and the presence of 1.4 Ga orogenic granitoids in the CTB, rule out most cratons as the CTB sources but support a Fennoscandia ancestry. Zircon U‐Pb ages and Hf isotopic compositions from the CTB and Fennoscandia suggest that from 1.8 to 1.4 Ga, the εHf(t) values increased toward more positive values, consistent with an exterior orogen characteristic that the lower crust was replaced by a juvenile arc crust. In contrast, from 1.4 to 0.9 Ga, zircon εHf(t) values decreased to more negative values, reflecting an interior orogen, characterized by enhanced contribution of recycled crustal material from collided continental fragments. This marked shift most likely reflected a transition from breakup of Nuna to assembly of Rodinia, accomplished by a transformation from an exterior orogen to an interior one.

Caledonian metamorphism of metasediments from Franz Josef Land

ABSTRACTThe petrography, detrital zircon age distribution and 40Ar–39Ar ages of three samples from the Nagurskaya drill core of westernmost Franz Josef Land have been analyzed and are compared to similar rocks from the Lomonosov Ridge. The analyzed rocks, from near the base of the drill hole, consist of fine-grained psammitic to semipelitic schists, metamorphosed under greenschist-facies conditions. They are lithologically very similar to the recently analyzed metasediments from the Lomonosov Ridge. The detrital zircon age spectra of the samples from both the Lomonosov Ridge and beneath Franz Josef Land span the Meso- to Palaeoproterozoic with a main peak around 1.6 Ga, similar to lower Neoproterozoic metasedimentary rocks in East Greenland, on Svalbard and northern Norway, as well as from Cambrian sediments in Estonia and Silurian–Devonian sediments on Novaya Zemlya. Biotite in metasedimentary rocks from the Nagurskaya drill core indicate an Early Devonian 40Ar–39Ar metamorphic age of c. 400 Ma, possibly superimposed on an earlier, Ordovician history similar to that of the metasediments from the Lomonosov Ridge at 470 Ma. Previously published 40Ar–39Ar analyses of Nagurskaya muscovite yielded c. 600 Ma ages, characteristic of the Timanian Orogen. Together with the new biotite data, these isotope ages favor the interpretation that the Caledonian suture is located between Svalbard and Franz Josef Land, and the Caledonian deformation front between the latter and Novaya Zemlya.

Precambrian crustal evolution of the southwestern Tarim Craton, NW China: Constraints from new detrital zircon ages and Hf isotopic data of the Neoproterozoic metasedimentary rocks

Neoproterozoic metavolcanic-sedimentary sequences are well preserved along the southwestern margin of Tarim Craton, NW China. In order to constrain the depositional timing of the metasedimentary sequences and better understand the crustal evolution, we present new detrital zircon U–Pb ages and Hf isotopic data for the Neoproterozoic metasediments collected from the eastern Aliankate Group in the southwestern Tarim Craton (STC). The detrital zircons are characterized by high Th/U ratios, clear concentric zoning and euhedral shapes, which imply that the metasediments of the Aliankate Group were mainly derived from a proximal magmatic source. The U–Pb dating of the detrital zircons from three samples shows similar age spectra with an overwhelming population of early to middle Neoproterozoic ages. Zircons from this Neoproterozoic population of the three samples yielded uniform youngest peak age at 790–808 Ma, respectively, indicating the maximum depositional age of the eastern Aliankate Group. Based on the published age data of detrital zircons, we suggest that the Aliankate Group most likely deposited at 776–790 Ma. Because of the slightly different main age peaks, we also suggest that the Neoproterozoic source materials for the eastern and western of this group may be different although they were both derived from a Neoproterozoic source. The crustal Hf model ages of the Aliankate detrital zircons yield three distinct populations with peaks at 2.13–2.20 Ga, ~2.81 Ga, and ~3.03 Ga, respectively. In combination with the previous published data, we suggest that the southwestern Tarim Craton experienced similar crustal evolution to the northern in the Archean, especially for the earliest three crustal growth events at ~3.6 Ga, ~3.0 Ga, and ~2.8 Ga, but much different since the early Paleoproterozoic. Therefore, we suggest that the southwestern and northern Tarim Craton have uniform Archean basement and likely separated from each other in the early Paleoproterozoic.

Precambrian basement, provenance implication, and tectonic evolution of the Gargan block of the Tuva-Mongolia terranes, Central Asian Orogenic Belt

Although the Phanerozoic geology of the Central Asian Orogenic Belt is extensively studied, there is still much to reveal about the initiation and early stages of tectonism. The Gargan block of the Tuva-Mongolia terranes at the Mongolian-Russian border is one of the Archean-Proterozoic continental crustal fragments that record the earliest evolutionary history of the belt. To better constrain the age of its basement, extent, and provenance in regional tectonic framework, we present new zircon UPb geochronologic data from previously undated basement gneisses and the overlying Neoproterozoic meta-sedimentary rocks of southern part of the Gargan block. Comparison of available basement ages and Precambrian detrital zircon spectra from nearby Archean-Proterozoic continental crustal fragments show complex relationship amongst them. We propose that the Gargan block travelled by itself in most of the Proterozoic or was derived from an unknown continental block. Obtained age confirms the existence of Neoarchean-Neoproterozoic basement in the region; however, for the first time, the 2749–814 Ma Salig Complex reveals multiple stages of Neoproterozoic metamorphic events within the Central Asian Orogenic Belt. Older of which, at ∼1 Ga, may relate to an assembly of several of the pre-existing continental crustal fragments in Mongolia.

Variations of the 238U/235U isotope ratio in metasedimentary rocks and evidence of changes in sedimentation conditions during the Ediacaran period of the Neoproterozoic

The isotopic composition of uranium is a new geochemical indicator that facilitates reconstruction of the redox conditions of geological processes. In this paper the results of study of the 238U/235U isotope ratio in the Neoproterozoic metasedimentary rocks of the Baikal–Patom fold belt (Northern Transbaikalia) were obtained using the MC-ICP-MS method and 233U+236U double spike. The scale of 238U/235U ratio variations (δ238U = –0.37 to –0.11‰) is within the range of (–0.39 to +0.15‰), typical of metasedimentary terrigenous rocks of marine origin. In terms of the δ238U values, uranium of the BPB rocks studied is heavier than seawater uranium (δ238U = –0.41 ± 0.03). There is a correlation between the δ238U value in the rocks and their position in the stratigraphic section. Variations of the 238U/235U ratio in the terrigenous-carbonate sequences of BPB indicate that the sedimentation conditions changed and euxinic conditions were established in the course of evolution of the paleobasin in the Late Ediacarian Period.

Geochemistry and provenance of Neoproterozoic metasedimentary rocks from the Togo structural unit, Southeastern Ghana

Neoproterozoic metasedimentary rocks of greenshist facies, consisting predominantly of quartzites, phyllites, and phyllonites, occur in the Akwapim range of the Togo Structural Unit (TSU) in Ghana. The geochemistry of the phyllites were studied to determine their provenance and tectonic setting. The major element analysis was carried out by the Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) and trace elements, including REEs, by the Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS) method. The studied metasedimentary rocks have SiO2 and Al2O3 contents comparable to that of average Neoproterozoic upper crust. The metasedimentary rocks are strongly depleted in CaO, Na2O, and Sr and enriched in K2O, Ba and Rb with respect to average Neoproterozoic upper crust, reflecting K addition during diagenesis. Zr and Hf concentrations are significantly above Neoproterozoic upper crustal values. Chondrite-normalized rare earth element (REE) patterns are characterised by fractionated light-REE (LREE) (average LaN/SmN = 3.68), significant negative europium anomaly (average Eu/Eu* = 0.61) and fairly flat heavy-REE (HREE) (average GdN/YbN = 1.3). The geochemical data, particularly the high La/Sc, Th/Sc, La/Co, Th/Co, Zr/Sc, La/Th ratios, Eu/Eu* values, and high Zr and Hf concentrations, suggest that the metasedimentary rocks of the Togo Structural Units in the Akwapim range were derived mainly from recycled sedimentary sources. Comparison of geochemical signatures of the studied metasedimentary rocks with those of the Paleoproterozoic Birimian rocks suggests that the felsic materials in the Togo Structural Units (TSU) could not have been derived predominantly from the Birimian rocks, implying more distal sources. The studied metasedimentary rocks exhibit provenance characteristics similar to that of the Kwahu/Bombouaka Group of the Voltaian Basin, suggesting derivation from the same source, probably in the Amazon Craton. The metasedimentary rocks exhibit geochemical characteristics indicative of sediments derived from a passive continental margin. This inference supports previous studies that indicate that prior to the Pan-African Dahomeyide orogenic event, the southern margin of the West African Craton (WAC) was under passive margin settings.

Variations of the 238U/235U Isotope Ratio in Metasedimentary Rocks and Evidence of Changes in Sedimentation Conditions during the Ediacarian Period of the Neoproterozoic

The isotopic composition of uranium is a new geochemical indicator that facilitates reconstruction of the redox conditions of geological processes. In this paper the results of study of the 238 U/ 235 U isotope ratio in the Neoproterozoic metasedimentary rocks of the Baikal–Patom fold belt (Northern Transbaikalia) were obtained using the MC-ICP-MS method and 233 U+ 236 U double spike. The scale of 238 U/ 235 U ratio variations (δ 238 U = –0.37 to –0.11‰) is within the range of (–0.39 to +0.15‰), typical of metasedimentary terrigenous rocks of marine origin. In terms of the δ 238 U values, uranium of the BPB rocks studied is heavier than seawater uranium (δ 238 U = –0.41 ± 0.03). There is a correlation between the δ 238 U value in the rocks and their position in the stratigraphic section. Variations of the 238 U/ 235 U ratio in the terrigenous-carbonate sequences of BPB indicate that the sedimentation conditions changed and euxinic conditions were established in the course of evolution of the paleobasin in the Late Ediacarian Period.

Cryogenian glaciomarine megaclasts of the MacDonald Corridor, Bimbowrie Conservation Park, Olary Region, South Australia

Ten glacially related megaclasts, unique perhaps from a global perspective, have been identified in the MacDonald Corridor. Included are perhaps the largest known glacially-related basement-derived megaclasts, the Bimbowrie Megaclast measuring 1.25 km in exposed width. The MacDonald Corridor is the most outward of four half-grabens to the east of the main Adelaide Geosyncline rift complex. All four contain Neoproterozoic low-grade metasedimentary rocks situated between inliers of Paleoproterozoic–Mesoproterozoic basement at the southwestern margin of the Curnamona Province. These corridors originated as extensional structures during the last of three major episodes of rifting that led to the eventual break-up of the supercontinent Rodinia. This rift phase was synchronous with the older (Sturtian-age) of the two major Cryogenian glaciations that are recorded in the rift and sag-basin complex of the Adelaide Geosyncline in South Australia. Those megaclasts selected for description vary in size from 0.1 to 1.25 km in exposed width, and were derived from the eastern rift flank, the Kalabity Inlier They therefore comprise either Paleoproterozoic metamorphic or Mesoproterozoic rocks or a composite of both. The megaclasts fall into two groups, those contained within the sedimentary fill of the rift basin, the Bimbowrie and Adlaweena megaclasts being the largest, and those resting as erratics on the flank. Of these, the 450 m wide Old Boolcoomata Megaclast resides within the remains of a steeply dipping U-shaped glacial valley. Tectonic activity and ice action are considered to have been responsible for creating the high-relief topography of the Kalabity Inlier. Ice was also instrumental in separating the megaclasts from their source and transporting them into the tectonically active MacDonald Corridor half-graben. Additionally, the variation of floating ice-cover controlled iron deposition within the glacigene Benda Siltstone. The present morphology of the MacDonald Corridor is largely due to the Cambrian Delamerian Orogeny, which while causing some structural complexity, exhumed and tilted the infill of the corridor to view.

Precambrian crustal evolution of the central Jiangnan Orogen (South China): Evidence from detrital zircon U-Pb ages and Hf isotopic compositions of Neoproterozoic metasedimentary rocks

The Neoproterozoic amalgamation and timing of collision between the Yangtze and Cathaysia sub-blocks along the Jiangnan Orogen in South China remain disputed. With a view to constrain the crustal evolution in this major orogen, here we present new results from petrology, geochemistry, zircon U-Pb chronology and Lu-Hf isotopes, on a suite of meta-sedimentary rocks from the Shuangqiaoshan Group and tuffaceous rock from the Liantuo Formation in the Jiuling terrane of central Jiangnan Orogen. Magmatic zircon grains in meta-sedimentary and sedimentary samples from the bottom to top part of the lower and upper Shuangqiaoshan groups constraints the deposition during 863–820 Ma and 797–780 Ma, respectively. The unconformities were formed during 820–797 Ma, reflecting the final amalgamation of Yangtze with Cathaysia sub-blocks along the central Jiangnan Orogen. Our data also identify four major events at 1000–800, 1500–1900, 1900–2200, 2300–2600 Ma. The Neoproterozoic detrital zircons in the lower Shuangqiaoshan Group possess variable εHf(t) values of −44.0 to +13.4. Combined with the geochemical features, these data suggest that the lower Shuangqiaoshan Group was deposited in a back-arc basin with a mixed provenance of granitic and felsic volcanic components, whereas the upper Shuangqiaoshan Group was formed in a rift basin with more granitic or felsic components in the source region. Integrating the results presented in this study with those from previous works, a five-stage crustal evolution model is proposed for the Neoproterozoic Jiangnan Orogen involving (1) intra-oceanic subduction beneath the Huaiyu oceanic island arc (1000–880 Ma), (2) collision between Huaiyu arc and Yangtze Sub-block (880–860 Ma), (3) back-arc extension beneath the Yangtze Sub-block (860–830 Ma), (4) collision between Cathaysia and Yangtze sub-blocks (830–800) and (5) intracontinental rifting throughout South China Block (800–750 Ma).

Evidence for an Early Silurian Synorogenic Basin Within the Metamorphic Hinterland of the North Atlantic Caledonides: Insights From the U‐Pb Zircon Geochronology of the Funzie Conglomerate, Shetland, Scotland

AbstractThe spectacular kilometer‐thick Funzie Conglomerate is a deformed and metamorphosed high‐energy sedimentary deposit that occurs in Shetland (Scottish Caledonides). Its age and tectonic significance in relation to Early Ordovician (485–475 Ma) and Late Ordovician (450–445 Ma) accretionary orogenic events and the culminating Silurian (430–425 Ma) continental collision and closure of the Iapetus Ocean have hitherto been unknown. U‐Pb zircon dating indicates that the clast population of the conglomerate was derived from easterly sources comprising Neoproterozoic metasedimentary rocks and the northern extension of the Ordovician‐Silurian Midland Valley Arc that formed during Iapetus closure. A lower age limit of 440 Ma is indicated by the ages of the youngest matrix detrital zircon grains and the youngest granitic clast. Deformation and metamorphism of the conglomerate is attributed to sinistrally oblique extension at ca. 430 Ma that excised at least 10 km of crustal section. Sedimentation of the conglomerate is therefore constrained to a 10‐myr time span (440–430 Ma). The tectonic setting is interpreted as a synorogenic basin that received detritus from, and was buried by, advancing thrust nappes during the Early Silurian collision of Laurentia and Baltica. Tectonic burial was followed rapidly by ductile deformation during oblique extensional unroofing of the metamorphic pile and accounts for preservation of the Funzie Conglomerate as a rare example of a synorogenic basin within the metamorphic hinterland of the North Atlantic Caledonides.

A Cambrian mixed carbonate–siliciclastic platform in SW Gondwana: evidence from the Western Sierras Pampeanas (Argentina) and implications for the early Paleozoic paleogeography of the proto-Andean margin

The Western Sierras Pampeanas (WSP) of Argentina record a protracted geological history from the Mesoproterozoic assembly of the Rodinia supercontinent to the early Paleozoic tectonic evolution of SW Gondwana. Two well-known orogenies took place at the proto-Andean margin of Gondwana in the Cambrian and the Ordovician, i.e., the Pampean (545–520 Ma) and Famatinian (490–440 Ma) orogenies, respectively. Between them, an extensive continental platform was developed, where mixed carbonate–siliciclastic sedimentation occurred. This platform was later involved in the Famatinian orogeny when it underwent penetrative deformation and metamorphism. The platform apparently extended from Patagonia to northwestern Argentina and the Eastern Sierras Pampeanas, and has probable equivalents in SW Africa, Peru, and Bolivia. The WSP record the outer (deepest) part of the platform, where carbonates were deposited in addition to siliciclastic sediments. Detrital zircon U–Pb SHRIMP ages from clastic metasedimentary successions and Sr-isotope compositions of marbles from the WSP suggest depositional ages between ca. 525 and 490 Ma. The detrital zircon age patterns further suggest that clastic sedimentation took place in two stages. The first was sourced mainly from re-working of the underlying Neoproterozoic metasedimentary rocks and the uplifted core of the early Cambrian Pampean orogen, without input from the Paleoproterozoic Rio de la Plata craton. Sediments of the second stage resulted from the erosion of the still emerged Pampean belt and the Neoproterozoic Brasiliano orogen in the NE with some contribution from the Rio de la Plata craton. An important conclusion is that the WSP basement was already part of SW Gondwana in the early Cambrian, and not part of the exotic Precordillera/Cuyania terrane, as was previously thought.

Isotopic (Sm–Nd, Pb–Pb, and δ34S) and Geochemical Characteristics of the Metasedimentary Rocks of the Baikal–Patom Belt (Northern Transbaikalia) and Evolution of the Sedimentary Basin in the Neoproterozoic

This paper reports the results of a detailed isotopic (Sm–Nd, Pb–Pb, and δ34S) and geochemical studies of Neoproterozoic metasedimentary rocks from the Patom and Bodaibo domains of the Baikal–Patom belt (northern Transbaikalia). It was shown that the metasedimentary rocks of these domains are strongly variable in their geochemical and isotope geochemical characteristics. Regular variations in these characteristics were observed, and their correlation with the main stages of the evolution of the sedimentary paleobasin in the Neoproterozoic was established.

Partial-melting of fertile metasedimentary rocks controlling the ore formation in the Jiangnan porphyry-skarn tungsten belt, south China: A case study at the giant Zhuxi W-Cu skarn deposit

The Zhuxi W-Cu deposit, located in the Jiangnan porphyry-skarn W belt, is a world-class W deposit. We studied three coeval mineralization-related intrusions composed of biotite monzogranite, fine-grained granite, and granite porphyry in the Zhuxi mine. These rocks contain peritectic garnet and K-feldspar. The LA-ICP-MS U–Pb dating of zircon from the biotite monzogranite, fine-grained granite, and granite porphyry yields average ages of 149.38 ± 0.86 Ma, 149.0 ± 1.0 Ma, and 148.30 ± 1.4 Ma, respectively. The Zhuxi granites are enriched in Cs, Rb, and U and depleted in Ba, Sr, and Ti, with ASI [molar Al2O3 / (CaO + Na2O + K2O)] values of 1.03–2.15. The fine-grained granite exhibits initial 87Sr/86Sr values of 0.716–0.717 and εNd(t) values ranging from −9.61 to −9.21. The εHf(t) values of the biotite monzogranite and fine-grained granite range from −8.83 to −6.30 and from −9.86 to −7.62, respectively. The Sr–Nd–Hf isotopic compositions of these rocks are similar to those of the fertile Neoproterozoic metasedimentary rocks in the Jiangnan W belt.The Zhuxi granites are S-type granites based on their mineral assemblages and geochemical characteristics. The Hf isotopic compositions, Sr–Nd isotopic characteristics, and trace element modelling suggest that the studied granites formed from the dehydration melting of fertile Neoproterozoic metasedimentary rocks caused by the Late Jurassic underplating of OIB-like basaltic magma.

P-T evolution of metasedimentary rocks of the Santa Filomena Complex, Riacho do Pontal Orogen, Borborema Province (NE Brazil): Geothermobarometry and metamorphic modelling

We present in this paper petrologic data and discuss the pressure-temperature (P-T) metamorphic history of the neoproterozoic metasedimentary rocks of the Santa Filomena Complex, Riacho do Pontal Orogen, which is inserted in the southern portion of the Borborema Province (Northeast Brazil). Therefore, the data provide constraints on metamorphic evolution during Neoproterozoic Brasiliano Orogeny in Northeast Brazil. The rocks studied are aluminous schists and paragneisses. Silver-gray and red pelitic schists are intensely deformed, biotite-muscovite rich, contain centimeter-sized garnet, staurolite and kyanite porphyroblasts, and subordinately plagioclase and quartz. Paragneisses are from light gray to dark gray colored, medium to coarse-grained and display a well-spaced foliated matrix of biotite, and kyanite and garnet porphyroblasts. Locally, the schists and paragneisses are migmatized. Pressure-temperature modelling based on thermobarometric calculations indicate that metamorphism reached 643 °C with pressures estimated in 12 kbar. Pre-peak and post-peak metamorphic conditions are constrained by mineralogical and textural relationships: garnet inclusion-rich and inclusion-free (possible of higher T) are documented and the inclusion-rich core probably indicates a Sn-1 foliation that was transposed by Sn. The pre-peak stage most probably occurred close to 500 °C and 8 kbar, in upper greenschist to lower amphibolite facies metamorphism along kyanite stability field. We also propose that post-peak stage was associated with isothermal decompression along a possible path of tectonic exhumation in conditions of 600 °C and 7 kbar. To further evaluate the equilibrium condition, pressure-temperature pseudosections were calculated for the metasedimentary rocks. Thus, the estimated metamorphic peak took place in the upper amphibolite facies. A suggested clockwise pressure-temperature path is compatible with the regional tectonic setting of continent-continent collision which occurred in the Late Neoproterozoic of Borborema Province, during the Brasiliano Orogeny.

Geochronological and geochemical constraints on sulfide mineralization in the Qingmingshan mafic intrusion in the western part of the Proterozoic Jiangnan orogenic belt along the southern margin of the Yangtze Craton

The Qingmingshan magmatic Fe-Ni-Cu sulfide deposit is located in the southern margin of the Yangtze Craton, in the western part of the Proterozoic Jiangnan orogenic belt. Sulfide mineralization occurs as disseminated zones in a dike-like mafic body intruding Early Neoproterozoic meta-sedimentary rocks. The host intrusion can be further divided into an upper gabbronorite unit, a middle dolerite unit and a lower gabbronorite unit. Each unit contains at least one disseminated sulfide zone. Sulfide mineral assemblages are composed of pyrrhotite, pentlandite and chalcopyrite. Zircon U–Pb dating yields a crystallization age of 847.8±3.8Ma (2σ) for the intrusion. The host rocks are characterized by enrichment in light rare earth elements (REE) relative to heavy REE and show pronounced negative Nb-Ta anomalies, similar to the characteristics of arc mafic igneous rocks worldwide. Mixing calculations indicate that the variations of (Th/Yb)N and (Th/Nb)N in the intrusion are consistent with between 10 and 20wt% crustal contamination of the parental magma. The sulfide separates are characterized by elevated δ34S values from 1.5 to 3.5‰, with a peak at 2.6‰, and by γOs values from 2 to 89, indicating addition of crustal sulfur and Os to the parental magma. The trace element data and S-Os isotopes support the premise that sulfide saturation in the Qingmingshan magma was triggered by crustal contamination. Variations of PGE tenors in different sulfide zones can be explained by variable R-factors from 300 to 1000 during sulfide segregation from the magma. Mass balance calculations indicate that the parental magma was severely depleted in PGE, which could be a primary signature of arc basaltic magma formed by low degrees of partial melting or due to previous sulfide segregation at depth. The lithological and mineralization zonation across the dike-like Qingmingshan mafic intrusion indicates that at least three separate pulses of sulfide-charged magma were involved in the formation of the Qingmingshan magmatic sulfide deposit. The Qingmingshan mafic intrusion is interpreted to be a part of a sub-arc magma plumbing system. Coeval ultramafic intrusions formed from more primitive magma in the region are good exploration targets for magmatic sulfide deposits with higher Ni/Cu ratios than the Qingmingshan deposit.

Basement components of the Xiangshan-Yuhuashan area, South China: Defining the boundary between the Yangtze and Cathaysia blocks

The exposed metamorphic rocks and xenolithic metamorphic rocks in the Xiangshan-Yuhuashan (XSYHS) area in central Jiangxi Province, South China, were studied in order to reveal the compositions of the basement rocks and their tectonic implications. Bulk geochemistry and zircon U-Pb-Hf isotopic compositions suggest that the basement metamorphic rocks in the XSYHS area are mainly composed of Neoproterozoic (after 822–683Ma) meta-sedimentary rocks. The exposed meta-sedimentary rocks show higher K2O/Na2O and Al2O3/(Na2O+CaO) and lower Eu/Eu∗ than those occurring as xenoliths in the Yuhuashan volcanic-intrusive complex, indicating relatively strong weathering and leaching in the provenance area. Geochemical data suggest that the detritus of the exposed metasedimentary rocks was derived mainly from felsic to intermediate igneous rocks, similar to Neoproterozoic sedimentary rocks in the southern Yangtze Block, whereas the xenolithic metasedimentary rocks contain more recycled material, similar to those in the Cathaysia Block. These xenoliths from the mid-lower crust also have significantly high U contents. Consequently they probably are an important source for uranium in the XSYHS volcanic rocks.U-Pb ages of detrital zircons show that the protoliths of the xenoliths are mainly composed of Neoproterozoic, Neoarchean and Paleoproterozoic detritus, similar to the components of the Wuyi terrane in the eastern Cathaysia Block. In contrast, the protoliths of the exposed metasedimentary rocks consist mainly of Neoproterozoic detritus with minor Neoarchean and Paleoproterozoic fragments, similar to those in the southern Yangtze Block. Both geochemistry and zircon U-Pb-Hf isotope compositions suggest that the deep basement metamorphic rocks in the XSYHS area belong to the Wuyi terrane, whereas the surface metamorphic rocks are derived from the southern Yangtze Block. Therefore, the real boundary between the Yangtze and Cathaysia blocks lies north of the study area. The exposed Yangtze-related metamorphic rocks were overthrust southward from the southern Yangtze Block, and onto the Cathaysia basement, during the Indosinian to Yanshanian Orogeny.