Zhen-Yu Luo

Sedimentation and Lithofacies Paleogeography in Southwestern China Before and After the Emeishan Flood Volcanism: New Insights into Surface Response to Mantle Plume Activity

Investigations into Permian sedimentation and reconstruction of paleogeography in SW China are aimed at characterizing sedimentary responses to the Emeishan mantle plume. In addition to erosional features on the sediments underlying the uplifted Emeishan basalts, unusual depositions of Permian age are also present in the Emeishan large igneous province (LIP). Specifically, carbonate gravity flows and submarine incised canyon fillings were developed in the western margin of the postulated uplifted area, and rifting trenches were developed along the eastern margin; alluvial fan deposits occur at the boundary between the inner and intermediate zones. These depositions all rest on the Maokou Formation and are in turn covered by the Emeishan basalts, implying synchronism between crustal uplift and depositional events. These deposits and the associated extension and normal faulting along the margin of and within the LIP represent sedimentary features resulting from dynamic behavior of mantle plume. Comparison of lithofacies paleogeography before and after the Emeishan flood volcanism highlights the determinant role of mantle plume activity in the geological evolution in SW China. The rapid, differential erosion of the Maokou Formation was likely related to plume‐induced dynamic uplift. This uplift was apparently followed by subsidence, given deposition of the marine clastic rocks sandwiched between basalts and the Maokou Formation in the east and submarine basalts along the margins of the province. A second‐phase uplift, attributed to underplating of plume‐derived melts at the crust‐mantle boundary, was characterized by prolonged (∼45 m.yr.), plateau‐type uplift and was responsible for the appearance of the “Chuandian old land.” Integration of these erosional and depositional characteristics allows us to depict how the surface geology responds to mantle plume, which explains some complex sedimentological problems in SW China.

The Imandra/Varzuga greenstone belt

The Late Archaean-Early Palaeoproterozoic transition (2500–2000 Ma) represents a hallmark period when the Earth System experienced a series of fundamental upheavals. Among them, the most important was the establishment of an oxygen-rich atmosphere (sometimes referred to as the Great Oxidation Event) and the emergence of an aerobic biosphere. Associated with this, either incidentally or causally, was a cascade of other prominent, global-scale events that considerably modified Earth’s surface environments, either temporarily or permanently; these are reviewed in Parts 1 and 8 in full, and detailed in Part 7. Briefly mentioned here, these include: the severe and global climatic event known as the Huronian glaciation; an unprecedented perturbation of the global carbon cycle, the large-magnitude Lomagundi-Jatuli positive excursion of δ13Ccarb, lasted over 160 Ma; radical changes in the phosphorus and sulphur cycles resulting in accumulation of the first-known massive sulphates and sedimentary phosphates; a radical modification in recycling of organic matter leading to the emergence of a new 13C-depleted carbon reservoir in the form of carbonate concretions; and an unprecedented accumulation of organic-rich sediments and formation of the earliest supergiant petroleum deposits.

Origin of high-An plagioclase in the early Permian (~280 Ma) Xiaohaizi wehrlite, Northwest China: insights from melt inclusions in clinopyroxene macrocrysts and zircon oxygen isotopes

ABSTRACT The Xiaohaizi wehrlite intrusion in the early Permian Tarim Large Igneous Province, Northwest China, is characterized by unusual high-An (up to 86) plagioclases. It has been suggested that H2O may have exerted a major control on their formation, but this interpretation requires further direct evidence. Moreover, it remains unclear where the water came from. In order to unravel these questions, we present electron microprobe analyses of minerals and melt inclusions in clinopyroxene macrocrysts in the dikes crosscutting the Xiaohaizi wehrlite intrusion and in situ oxygen isotope data of zircons from the Xiaohaizi wehrlite. The homogenized melt inclusions have restricted SiO2 (45.5–48.7 wt.%) and Na2O + K2O (2.4–3.8 wt.%) contents, displaying sub-alkaline affinity. This is inconsistent with the alkaline characteristic of the parental magma of the clinopyroxenes, suggesting significant modification of melt inclusions by contamination of the host clinopyroxene due to overheating. Nevertheless, the Ca/Na ratios (2.9–4.7) of melt inclusions are the upper limit of the parental magma of the clinopyroxenes due to high CaO (21.5–23.0 wt.%) and very low Na2O (0.22–0.34 wt.%) contents in the host clinopyroxenes. Thermodynamic calculation suggests that under fixed P (2.7 kbar) and T (1000°C), and assumed H2O (~1.5 wt.%) conditions, the Ca/Na ratio of the parental magma cannot generate high-An plagioclase in the wehrlite. The results confirm that H2O exerts a major control. Zircon δ18O (VSMOW) values (2.99–3.71‰) are significantly lower than that of mantle-derived zircon (5.3 ± 0.6‰). Such low zircon δ18O values may be due to incorporation of large amounts of low-δ18O, hydrothermally altered oceanic crust. However, geochemical and Sr-Nd-Pb isotopic data do not support recycled oceanic crust in the mantle source of the Xiaohaizi intrusion. Alternatively this can be explained by incorporation of meteoritic water in the magma chamber. This will increase the H2O content of the liquid that finally crystallize high-An plagioclases.