A Geochemical Study Of Late Mesozoic Volcanics From The Middle-lower Yangtze Valley

Plate tectonics theory well explains the origin of arc volcanic rocks at modern convergent plate margins. What remains to be resolved are the nature of mantle sources for mafic arc volcanics, the mechanism for incorporation of subducted slab components into the mantle sources, and the geochemical differentiation of primitive arc magmas during their ascent prior to eruption on the continental crust. On the other hand, mafic igneous rocks with arc-like geochemistry are common in the continental interior. Because they are far away from modern subduction zones, it is unclear whether such magmatism is controlled by the modern plate tectonics or the ancient tectonic event. This PhD thesis deals with the petrogenesis of Late Mesozoic volcanics in the Middle-Lower Yangtze Valley of South China by a combined study of geochronology, petrology and geochemistry. The results not only present a perspective on the pertrogenesis of Late Mesozoic volcanics in the Middle-Lower Yangtze Valley, but also establish the relationship between crust-mantle interaction in the ancient oceanic subduction zone and the arc-like magmatism in the modern continental interior.Chapters 1 to 4 of this thesis present introduction to the background of this study (Chapter 1), the geological setting of the studying region (Chapter 2), the description of samples (Chapter 3) and analytical methods that were used in this study (Chapter 4). Chapter 5 addresses the origin of mantle sources above the fossil Andes-type oceanic subduction zone and the petrogenesis of continental arc andesites through a combined study of whole-rock major-trace elements and Sr-Nd-Pb-Hf isotopes as well as zircon U-Pb ages and Hf-O isotopes, specifically for basaltic-andesitic volcanics from the Luzong basin. In Chapter 6, a series of data for zircon U-Pb ages and Hf-O isotopes, and whole-rock major-trace elements and Sr-Nd-Hf isotopes is used to investigate whether the petrogenetic model of Luzong andesites is applicable to Late Mesozoic andesites elsewhere in the Middle-Lower Yangtze Valley. In Chapter 7, a combined petrological and geochemical study was made on clinopyroxene phenocrysts from the Ningwu volcanics and results are used to decipher magmatic processes in andesite petrogenesis and to evaluate the relative contributions of magma differentiation and source mixing to the geochemical composition of the andesite. In Chapter 8, a comprehensive study of zircon U-Pb ages and Hf-O isotopes, and whole-rock major-trace elements and Sr-Nd-Hf isotopes, and phenocryst major elements were carried out for bimodal volcaniscs in the Fanchang basin to decipher their origin. In Chapter 9, a compilation is made on published geochemical data on the Late Mesozoic volcanics in the Middle-Lower Yangtze Valley. After a comprehensive comparison of the geochemical compositions of basalts, andesites and dacites-rhyolites, an mtegrated interpretation is presented for the petrogenesis of these volcanics, with possible constraints on the tectonic setting of such magmatism. Based on the observation that the northeastern margin of the Jiangnan orogen was extended to the Middle-Lower Yangtze Valley, it is proposed that the crust-mantle interaction between slab-derived felsic melts and mantle wedge peridotite was operated in the fossil subduction channel during subduction of the Cathaysia oceanic lithosphere beneath the Yangtze Craton in the Early Neoproterozoic. In this regard, the nature of the Late-Mesozoic magmatism in the Middle-Lower Yangtze Valley can be viewed as reworking of the fertile and enriched lithospheric mantle in response to the regional lithospheric extension in the Late Mesozoic due to the westward subduction of Pacific slab beneath South China block.Late Mesozoic volcanics in the Luzong basin are primarily composed of basaltic trachyandesite and trachyandesite, with small amounts of trachybasalt and trachyte. They exhibit variable contents of SiCh (48.66-63.43 wt.%), MgO (0.39-4.85 wt.%), Na2O (1.22-6.07 wt.%) and K2O (2.53-10.10 wt.%), with highly variable K2O/Na2O ratios from 0.45 to 7.39. They are characterized by arc-like trace element distribution patterns, with significant enrichment of LILE, Pb and LREE but depletion of HFSE. They exhibit relatively enriched Sr-Nd-Pb-Hf isotope compositions, with initial 87Sr/86Sr ratios of 0.7050 to 0.7066, negative εNd(t) values of -8.0 to -3.1, negative εHf(t) values of -11.1 to -1.1, and elevated 207Pb/204Pb and 2O8Pd/2O4Pb ratios at given 206Pd/204Pb rattios. Zircon U-Pb dating yields consistent ages of 127±2 to 137±1 Ma for magma emplacement prior to volcanic eruption. The Zircon exhibits slightly high δ18O values of 5.3 to 7.6‰ and variable εHf(t) values of - 13.1 to 2.6. An integrated interpretation of all these geochemical data leads to the conclusion that the Luzong basaltic-andesitic volcanics were primarily derived from partial melting of fertile and enriched mantle sources that are similar to those of continental arc andesites. Such mantle sources are hypothesized to form by reaction of the mantle wedge peridotite with hydrous felsic melts derived from partial melting of subducting seafloor sediment due to the fossil Andes-type oceanic subduction. As a consequence, the mantle wedge would be metasomatized by larger amounts of the felsic melts than the case for common oceanic arc basalts. Therefore, the significant incorporation of sediment-derived felsic melts into the mantle wedge is a key to the origin of continental arc andesites.Late Mesozoic volcanics from the Ningwu basin are mainly composed of trachyandesite and trachyte, with subordinate of basaltic trachy-andesites. They exhibit variable and low MgO contents (0.11-3.73 wt.%) and low Mg# values (1.60-51.18), high K2O contents (1.99-9.88 wt.%). They are characterized by arc-like trace element distribution patterns, with significant enrichment in LILE and LREE but depletion in HFSE. Furthermore, these rocks exhibit relatively enriched whole-rock Sr-Nd-Hf-Pb isotope compositions, with initial 87Sr/86Sr ratios of 0.7050 to 0.7082, negative εNd(t) values of -8.1 to -1.8, negative to positive εHf(t) values of -6.7 to 0.39, and elevated 207Pb/204Pb (15.47-15.61) and 208Pb/204Pb ratios (37.75-38.44) at given 206Pb/204Pb ratios (17.76-18.28). Zircon U-Pb dating yields consistent ages of 128±2 to 133±2 Ma for magma emplacement prior to volcanic eruption. The zircon exhibits slightly high δ18O values of 5.7 to 7.8 ‰ and variable εHf(t) values of -5.8 to 0.7. These shoshonitic andesites can be categorized into two groups in view of their distinct compositional features. An integrated interpretation of their geochemical compositions leads to the conclusion that Group I volcanics were produced by a relatively larger extent of partial melting of more fertile and enriched mantle sources that contain a greater amount of phlogopite, whereas Group II volcanics were generated by a relatively smaller extent of partial melting of less fertile and enriched mantle sources that contain a less amount of phlogopite. In either case, these mantle sources are less ultramafic in lithochemistry due to reaction of metasediment-derived hydrous felsic melts with the mantle wedge peridotite. This metasomatism would take place during subduction of the Cathaysia oceanic crust beneath the Yangtze Craton in the Neoproterozoic, whereas the Mesozoic mafic magmatism in the Middle-Lower Yangtze Valley is essentially caused by reactivation of the fertile and enriched mantle sources due to lithospheric extension in response to the subduction of Pacific slab beneath the eastern China continent.A combined petrological and geochemical study were carried out for clinopyroxene phenocrysts from the Late Mesozoic volcanics in the Ningwu basin. The results not only tell us about magmatic processes in the petrogenesis of Ningwu volcanics, but also provide constraints on the relative contributions of magma differentiation and source mixing to the geochemical composition of these andesites. The phenocrysts show complex textural features and compositional zoning, in association with variable REE distribution patterns. Three types of clinopyroxene can be distinguished from their distinct REE distribution patterns. Type Ⅰ clinopyroxene shows smooth REE distribution patterns and significant negative Eu anomalies, Type Ⅱ clinopyroxene exhibits enrichment in LREE and MREE over HREE with moderate negative Eu anomalies, and Type III clinopyroxene shows depletion of MREE and significant negative Eu anomalies. The distinct compositional correlation patterns between the three types of clinopyroxenes suggest that these clinopyroxenes were crystallized from two kinds of primary melts. Among which, Type I clinopyroxene was crystallized from less REE-enriched primary melts, whereas Types Ⅱ and Ⅲ clinopyroxenes were crystallized at different evolution stages of the more REE-enriched primary melts. The transition from Type II clinopyroxene to Type III clinopyroxene indicates that large amounts of plagioclase and amphibole crystals were crystallized from the magmas. However, the host rocks of these phenocrysts show insignificant negative Eu anomalies and negligible MREE depletion. This suggests that the majority of plagioclase and amphibole antecrysts were not separated from the host magmas and thus have negligible contribution to the final composition of andesitic volcanics. The patchy and reverse zoned clinopyroxene phenocrysts clearly record the magma recharge processes that operated during the magma evolution. Such persistent magma recharge spans the magmaric circles of interest and may be responsible not only for the restricted composition of andesitics from a later formation compared to andesitics from an earlier formation but also for the restricted composition of the Ningwu andesitic rocks compared to the rest of Mesozoic volcanics in the Middle-Lower Yangtze ValleThe Late Mesozoic volcanics in the Fanchang basin are bimodal in composition, showing compositional affinities to their contemporaneous andesites. Therefore, examining the petrogenesis of these bimodal volcanics would not only help to understand this Late Mesozoic magmatic event in the target area, but also place further constraints on the petrogenesis of these andesitic rocks. Mafic volcanics are composed of basalt, trachybasalt and tephrite with basaltic composition, whereas felsic volcanics are composed of mostly trachyte and rhyolite and rare trachy-andesite with dacitic to rhyolitic composition. Geochronological data suggest that the bimodal volcanic activity occurred between ca.129 and ca.125 Ma. The mafic lavas are characterized by:(1) arc-like trace element distribution patterns with variable enrichment of fluid-mobile incompatible elements; (2) moderately enriched Sr-Nd-Hf isotope compositions with quite homogeneous (87Sr/86Sr)i ratios of 0.7065 to 0.7067, low εNd(t) values of -5.61 to -3.51 and low εHf(t) values of -3.01 to -2.73; (3) highly variable zircon εHf(t) values from -20.5 to 7.9. An integrated interpretation of these observations indicates that the mafic rocks were originated from enriched amphibole-bearing mantle sources that were previously metasomatized dominantly by slab-derived aqueous fluids with subordinate metasediments-derived hydrous melts. On the other hand, the felsic lavas show comparable but somewhat more variable (87Sr/86Sr)i ratios from 0.7060 to 0.7076, and more negative εNd(t) values of -8.26 to -6.62 and εHf(t) values of -8.01 to -7.77. Their zircons have variably negative zircon εHf(t) values of-17.9 to-1.8 and relatively high δ18O values of 6.3 to 9.2%o. These geochemical features indicate that the felsic rocks were derived from partial melting of a terrigenous metasediment. The mafic volcanics show isotopic affinities to the felsic volcanics (especially in the zircon Hf isotope compositions), suggesting a petrogenetic link between them. A common metasedimentary component is found in their magma sources. Therefore, the bimodal volcanics are interpreted to be produced by postcollisional reworking of the crustal and mantle rocks, respectively, in a fossil oceanic subduction channel.Although the new data from the present study have provided important constraints on the petrogenesis of Late Mesozoic andesites and bimodal volcanics in the Middle-Lower Yangtze Valley, the petrogenetic relationship between all these rocks remains to be clarified and their implications for the nature of the Late Mesozoic volcanism in this area merit be exploring. Bearing this in mind, a compilation of published geochemical data is made on all Late Mesozoic volcanics in the target area. The results are used to evaluate the nature and tectonic setting of this magmatic event through geochemical comparison in geochemical composition between the basalts, andesites and dacites-rhyolites. The andesites showe obviously positive Th and U anomalies, which are not observed in the basalts. The andesites and basalts have similar Sr-Pb isotope composition, whereas the andesites are systematically enriched in Nd-Hf isotope composition than the basalts. Compared with the basalts, the andesites have relatively lower Ba/Th ratios and higher (La/Sm)N ratios. All these compositional similarities and differences lead to the conclusion that the andesites were derived from partial melting of the less ultramafic mantle sources that were generated by reaction of the mantle wedge peridotite mainly with metasediment-derived felsic melts and subordinately with metabasalt-derived aqueous fluids, In contrast, the basalts were derived from partial melting of the ultramafic mantle sources that were generated by reaction of the mantle wedge peridotite mainly with metabasalt-derived aqueous fluids and subordinately with metasediment-derived felsic melts. Compared with the basalts and andesites, the dacites and rhyolites have systematically more enriched Sr-Nd-Hf isotope compositions, which cannot be explained by AFC processes because of the lack of the correlation patterns between Sr-Nd-Hf-Pb isotopes and differentiation index. Combined with their enriched and highly variable zircon Hf isotope composition and high δ18O values, these geochemical features indicate that the felsic rocks were derived from partial melting of the terrigenous metasediment. Therefore, the petrogenesis of all the basalts, andesites and dacites-rhyolites in the Middle-Lower Yangtze Valley can be explained by postcollisional reworking of the variable mantle metasomatites and crustal rocks, respectively, in the fossil oceanic subduction channel that formed in Neoproterozoic, which was intrigged by the extension of regional lithospheric mantle and the upwelling of asthenoshperic mantle that caused by the westward subduction of Pacific plate beneath China continent in Late Mesozoic. In this regard, the petrogenetic model for arc-like volcanics in the Middle-Lower Yangtze Valley is applicable to common continental arc volcanics at modern active continental margins with respect to the formation of their mantle sources.