Precambrian Research最新文献

The origin of Earth’s felsic continental crust remains a mystery. The generation of felsic continental crust requires a two-stage partial melting from original mantle sources. There are two hypotheses for the continental crust generation in the early Earth. One is the subduction-related magmatism, e.g., island arcs, which produces intermediate to felsic magma that constitutes the early buoyant continental crust. The other is the magmatism induced by the mantle plume that creates thick basaltic crust and finally the continental crust. However, there is controversy about the origin of plate tectonics, which is an obstacle for simply applying the subduction-induced model in the early Earth. On the other hand, the efficiency of mantle plume-induced continental crust growth remains unknown. In this study, we develop a new numerical model, integrating the petrological-thermo-mechanical model with melt migration and crystallization, to evaluate the efficiency of continental crust production by mantle plumes in Earth’s history. Our results indicate that mantle plumes are considerably more effective for continental crust generation in the hot early Earth than that in the present Earth. The contribution of plume-induced continental crust growth may be greatly promoted by the possible high frequency of mantle plume generation in the early Earth than the present.

The special calcite bodies (CBs) exposed within the ultrahigh temperature (UHT) granulite facies terrain in the Balangoda area of Sri Lanka preserve shreds of evidence for the generation of crust-derived carbonatite in the crust. The CBs are exclusively made up of massive calcites and appear as concordant bands extending tens of meters or as meso-scale isolated pockets hosting the massive dolomitic marble band. Various sizes of mafic and calc-silicate enclaves occur as rotated or tilted structures within the CBs. The contact between the CBs and the host marble is texturally and mineralogically gradational, while the contact between the enclaves and the CBs is sharp. The large-ion lithophile elements and rare earth element contents of the CBs show enrichment compared to the host marble, while depletion compared to typical carbonatites. Furthermore, the Sr content and C-O isotope values in CBs differ from those found in known carbonatites, hydrothermal carbonates, or metasomatic carbonates. We suggested that the crustal anatexis of marble should be hypothesized as the possible mechanism for the origin of the CBs. Microtexural evidence of the calcite grains shows indications of the melting of the host marble. The release of CO₂-rich fluids from the collision and thrusting of HC over VC, or related metamorphic events, likely lowered the solidus of carbonates, triggering crustal anatexis of marble during UHT granulite facies metamorphism. The generated low viscous carbonate melt may have moved rapidly, resulting in a low degree of mixing of silicates and fragmentation and dislocation of enclaves. The results of the present study reflect the existence of anatexis of carbonates under extreme crustal conditions and provide a better understanding of the sources, migration paths and reservoirs of the carbon recycling processes.

Neoproterozoic granitic magmatism and associated mineralization in the western Yangtze Block (WYB), South China, records the assembly and breakup of the Rodinia supercontinent, making it a key window for understanding the tectonic evolution and associated dynamic process of the Rodinia supercontinent. In this paper, we present an integrated study on petrology, petrography, geochemistry, zircon U-Pb-Hf isotope, vesuvianite U-Pb and muscovite Rb-Sr geochronology for the W-rich granitic plutons (Xiacun and Shunhe plutons) and ore-bearing granitic dikes from the newly discovered Neoproterozoic Huili tungsten deposit in the WYB. Our objective is to provide vital constraint on the mineralization age and metallogenic geodynamic setting of the Neoproterozoic Huili tungsten deposit in the WYB. The ore body is characterized by skarn-type and quartz vein-type mineralization with main ore mineral of scheelite and gangue minerals of vesuvianite, muscovite and diopside. Zircon U-Pb dating indicates the granitic plutons and granitic dikes emplaced at ca. 845 Ma and 824 Ma, respectively. The vesuvianite U-Pb age and muscovite Rb-Sr isochron age are ca. 826–818 Ma and 822 Ma, respectively, indicating the tungsten mineralization was coeval with the crystallization of the granitic dikes. Petrology and geochemical studies reveal that the granitic dikes and the Shunhe pluton are highly fractionated S-type granite, while the Xiacun pluton belongs to unfractionated S-type granite. The granitic plutons have uniform and weakly enriched zircon Lu-Hf isotopes (εHf(t) = -3.08 ∼ -0.22), while the granitic dikes have varied zircon Hf isotopes (εHf(t) = -10.57 ∼ 4.64). Zircon Lu-Hf isotopes, as well as their varied whole-rock geochemical compositions, indicate that the Xiacun and Shunhe granitic plutons mainly originate from partial melting of Middle Paleoproterozoic crust, while the ore-bearing granitic dikes are mainly derived from the melting of heterogeneous Middle Paleoproterozoic crust, followed by significant fractional crystallization of feldspar, biotite and accessory minerals. Petrological, geochronological and geochemical data compilation reveals that the studied ca. 845 Ma granitic plutons and ca. 824 Ma W-mineralized granitic dikes were formed in a progressive subduction and a back-arc extension setting, respectively, and the Neoproterozoic back-arc extension-related granitic magmatism during the breakup of Rodinia supercontinent may be beneficial for the tungsten mineralization.

The late Tonian tectonic setting of the West Cathaysia terrane in South China is highly controversial and is key to better understanding the timing of amalgamation between the Yangtze Block and the West Cathaysia terrane and the positions of these two terranes in Rodinia and Gondwana. A detailed field-based study was conducted on the petrology, zircon U-Pb geochronology and Lu-Hf isotopes, and whole-rock geochemistry of a series of Neoproterozoic meta-volcanic and meta-sedimentary rocks in the Wuyi domain in the northeastern West Cathaysia terrane. The results indicate that the meta-volcanic rocks formed in an arc setting at 756-740 Ma, and the meta-sedimentary rocks were developed in an arc-related basin with single zircon age peaks of 765-728 Ma. We propose that the late Tonian arc in the Wuyi domain was part of a major arc system in the West Cathaysia terrane. An arc setting for the West Cathaysia terrane is distinct from the continental rift setting for the southeastern margin of the Yangtze Block at the time, supporting an interpretation that the two terranes were not amalgamated until after the late Tonian and occupied separate positions in Rodinia.

A plethora of proxies has been developed over the preceding two decades in attempts to investigate the geochemistry of the Archean ocean–atmosphere system, and in particular oxygen levels. Unfortunately the necessary parallel investigations of the effects that localized ocean chemistry and diagenesis can have on Archean sediments have commonly not kept pace. We used micro-analytical techniques (LA-ICP-MS and XRF scanning), to distinguish the effects of changes in water composition during precipitation and diagenesis on marine limestone precipitates at the margin and interior of a 2.8 Ga carbonate platform (Mosher Carbonate, Steep Rock Group) in western Superior Province, Canada. Platform margin meter-scale hybrid giant domes consist of centimetric interlayered couplets of (1) Sr-rich crystal fan fabric and cuspate fenestral microbialite, both with pronounced negative Ce-anomalies, and (2) net-like fenestral microbialite, rich in diagenetic cement with diminished concentrations of Sr and lacking negative Ce anomalies. The elevated Sr in the crystal fan fabric and cuspate fenestral microbialite is a general sign of less diagenetic alteration, as is preservation of millimeter-scale chemical differences. XRF mapping revealed that samples that otherwise appear pristine from a second site on the platform rim, near a zone of alteration in the limestone, have ferroan dolomite-filled micro-fractures with Mn flooding of the surrounding calcite and lack Ce anomalies. Platform interior silicified and ferroan dolomitized columnar stromatolites have some calcite laminae, but exceptionally low Sr contents indicate that they formed by dedolomitization. In several horizons REE patterns identical to those of offshore iron formation, replacement of calcite by iron carbonate, and the presence of iron oxides all suggest that short term flooding of the platform by offshore seawater episodically introduced ferroan dolomitizing fluids. Overall, these data indicate a restricted marine environment subject to periodic flooding by offshore waters that caused seafloor diagenetic alteration and precipitation of iron hydroxides. In this system the least altered limestone was the lithotype most likely to retain evidence of free oxygen. Samples with abundant phreatic cement and/or Mn alteration associated with micro-fractures were liable to have experienced REE mobilization. Detailed studies that integrate both depositional and diagenetic information are critical for the correct interpretation of geochemical data from sedimentary rocks.

The proposed genetic model for orogenic gold deposits hosted in Archean cratons worldwide has long assumed a single, late-orogenic mineralisation event. Recent geochronological developments and their application to gold deposits, however, have challenged this conventional view and indicate that gold endowment in several world-class Archean systems developed over distinct mineralisation stages. Here, we present U-Pb geochronological data from igneous zircons extracted from pre-, syn-, and post-mineralisation intrusions from the world-class Jundee gold deposit in the Yilgarn Craton of Western Australia. The crystallisation age obtained for a syn-mineralisation lamprophyre indicates that gold mineralisation occurred at ca. 2670 Ma, which is ca. 30 Ma older than the previously proposed gold episode (ca. 2640 – 2630 Ma) for the region. When integrated with fast-emerging, high-quality geochronological data from other major gold deposits across the Yilgarn Craton, our results highlight the occurrence of an early, syn-orogenic mineralisation event between ca. 2675 – 2655 Ma, which pre-dated the ca. 2640 – 2630 Ma late-orogenic gold event identified across the Kalgoorlie Terrane. This older gold mineralisation event was concurrent with the onset of the Neoarchean Kalgoorlie Orogeny in the Yilgarn Craton, which also coincided with a peak in mantle-derived magmatism (e.g., lamprophyres and sanukitoids). The younger gold event was coeval with intra-crustal granite plutonism and peak metamorphic conditions that occurred during the late- to post-collisional evolution of the Yilgarn Craton at ca. 2640 – 2630 Ma. Comparison between the Yilgarn Craton in Western Australia and the Superior Craton in Canada demonstrates that both record multiple gold events that occurred during protracted orogenesis. This study underscores the importance of tectonic processes in shaping the episodic nature of gold mineralisation in these ancient cratons.

Recent evidence from marine sedimentary rocks suggests that clay minerals were important in the supply of bioessential nutrients to seawater at the end of the Sturtian Snowball Earth glaciation that occurred ∼ 717–660 million years ago. However, little is known about the identity of these clay minerals, their abundance and distribution in the pre-glacial, glacial and post-glacial deposits. Here, high resolution petrographic scanning electron microscopy-energy dispersive x-ray spectroscopy, combined with bulk and < 2µ clay-size fraction X-ray diffraction mineralogical analysis and trace and major element geochemistry, point to dominant enrichment of illite in shallow marine platform sediments preserved across Sturtian Snowball Earth facies on Islay and the Garvellachs Island, Scotland. The glacial till is enriched with detrital chlorite, while pore-filling authigenic kaolinite is observed in the pre-glacial and post-glacial greenhouse facies, but scarce in the glaciogenic tillites. Discriminant binary plots of Th/Sc vs Zr/Sc ratios, indicative of insignificant modification of primary sediment composition by hydraulic sorting and recycling processes, together with chemical weathering indices, point to moderate to intense chemical weathering of the pre-glacial and post-glacial source rocks. Low Chemical Index of Alternation (CIA) values, coupled to the distinct chlorite composition of the glacial till, is consistent with mechanical tillage of continental landmass by the terminal Snowball melting ice sheets. The moderate to intense chemical weathering indices in the pre- and post-glacial deposits correspond with greenhouse conditions of elevated atmospheric CO₂ content and temperature. Overall, the predominantly detrital origin of the clay minerals implies enhanced post-glacial nutrient supply bound to clay minerals in lower pH river water during transportation, followed by release in higher pH saline seawater.

Amphibolite- to granulite-facies rocks are widely distributed in the Hengshan Complex, middle Trans-North China Orogen, and the high-pressure (HP) mafic granulite has been recently identified in the southern Hengshan area. The HP mafic granulite and amphibolite occur as rootless tectonic boudins/lenses within the TTG (tonalite–trondhjemite–granodiorite) gneiss/metapelite, indicative of typical “block-in-matrix” texture of metamorphic-tectonic mélange. Three to four generations of metamorphic mineral assemblages that correspond to the prograde (M1), peak (M2), and retrograde (M3-M4) stages, are recognized in these rocks. Conventional geothermobarometry and phase equilibrium modeling yield peak P–T conditions of 10.8–13.8 kbar/754–799 °C for the HP mafic granulite and 7.3–9.0 kbar/690–725 °C for the supracrustal rocks, respectively. A clockwise P–T path with near-isothermal decompression (ITD) and subsequent near-isobaric cooling (IBC) segments is reconstructed for the HP mafic granulite, indicating a dynamic subduction–collision–exhumation process that unfolded during an orogenic event. Secondary ion mass spectrometry (SIMS) U-Pb dating of zircon yields metamorphic ages of ca. 1.91–1.83 Ga, representing the long-lived tectono-metamorphic event caused by collision between the Eastern and Western Blocks along the Trans-North China Orogen. It is hypothesized that the tectono-metamorphic mélange in the area originated from the tectonically juxtaposition of metamorphic rocks that had diverse protoliths, different peak P–T conditions and discrepant metamorphic ages. This complexity may be a hallmark of Paleoproterozoic orogens, drawing intriguing parallels with the intricate characteristics observed in Phanerozoic orogenic belts.

The Neoarchean Qian’an BIF deposit in eastern Hebei, eastern North China Craton has attracted extensive attention of study in the last decades, but the genesis, e.g., Fe sources, metallogenic mechanism, as well as tectonic attributes of the BIFs remains highly disputed. Based on field investigations, microscopic observations and geochemical analysis, this study tries to unravel the source characteristics of ore-forming materials in the Qian’an deposit. The results show that the chemical compositions of the BIF ore samples are mainly composed of TFe₂O₃ and SiO₂, with minor Al₂O₃ and TiO₂. The total trace element contents of the samples are relatively low. The PAAS-normalized REE distribution patterns of the ores show LREE depletion and HREE enrichment, with robust positive anomalies of Eu, Y and La. These characteristics indicate that the BIFs are attributed to dominant chemical precipitation originated from paleo-ocean with obvious volcanic hydrothermal contributions and minor clastic input. Their positive to no Ce anomalies and positive δ⁵⁶Fe_magnetite values unveil that the iron was precipitated at low oxygen fugacity. These results, in collaboration with evidence from previous lithological, structural geology, metamorphic P-T paths, geochemistry, geochronology and numerical modeling studies, support a mantle plume model to explain the complicated tectono-thermal processes, and the sources of BIFs in Eastern Hebei, eastern North China Craton.

The Saglek Block in the Nain Province of northeast Canada is part of the North Atlantic Craton. It comprises Archean gneisses that record magmatic and metamorphic ages between ca. 3.9 and 2.5 Ga. In this study, a grey banded gneiss from Maidmonts Island records an age of 3.72 Ga, which is equated with Uivak I gneisses reported from across the Saglek Block, and contains 3.8 Ga xenocrysts. These rocks were deformed and metamorphosed prior to the intrusion of augen gneiss protoliths on both Maidmonts and Mentzel islands that record U-Pb zircon ages of 3.33 Ga. These rocks are composed of ferroan calc-alkaline granite and granodiorite and were likely generated by partial melting of pre-existing quartzo-feldspathic crust, as attested by the presence of ca. 3.8 Ga xenocrysts. Such augen gneiss was previously classified as ca. 3.6 Ga ‘Uivak II gneiss’, a term we argue should now be abandoned. The final magmatic events recorded on Maidmonts and Mentzel islands took place in the Neoproterozoic with the emplacement of granitic stocks at ca. 2.72 Ga, and sills and dykes at ca. 2.57 Ga. This sequence of magmatic events from the Eoarchean to Neoarchean is very similar to that recorded in the Itsaq Gneiss Complex of southwest Greenland, where, based on the Hf signature, it has been suggested that a change in tectonic environment resulted from the initiation of subduction at ca. 3.2 Ga. Although Lu-Hf and Sm-Nd isotopic signatures from the ca. 3.33 Ga gneisses have been interpreted in the literature as due to partial melting of Hadean mafic crust, alternatively, they can be generated by partial melting of Eoarchean continental crust in a continental arc setting. We argue this is more consistent with the non-TTG geochemistry of the augen gneisses.