Eruption of the Continental Flood Basalts at -259 Ma in the Emeishan Large Igneous Province, SW China: Evidence from Laser Microprobe ^(40)Ar/^(39)Ar Dating

A suite of continental flood basalts sampled over a vast exposure and stratigraphic thickness in the Emeishan large igneous province （LIP）, SW China was investigated for laser microprobe ^40Ar/^39Ar dating. There are two ^40Ar/^39Ar age groups for these basalts, corresponding to 259-246 Ma and 177-137 Ma, respectively. A well-defined isochron gives an eruption age of huge quantities of mafic magmas at 258.9±3.4 Ma, which is identical to previous dating and paleontological data. Much younger ^40Ar/^39Ar ages for some basalts with Iow-greenschist metamorphic facies probably recorded a late thermo-tectonic event caused by collision between the Yangtze and Qiangtang continental blocks during the Mesozoic, which resulted in the reset of argon isotope system. The ^40Ar/^39Ar age data, we present here, combined with previous dating and paleontological data, suggest relatively short duration （about 3 Ma） of mafic volcanism, which have important implication on mantle plume genesis of the Emeishan continental flood basalts in the LIP.

Petrology, Mineralogy and Geochemistry of the Emeishan Continental Flood Basalts, SW China: Evidence for Activity of Mantle Plumes

Electronic microprobe analyses for olivine, clinopyroxene and Cr-spinel in picrites, which we have discovered recently in the Emeishan continental flood basalt province (ECFBP), show that the olivine is rich in Mg, and that Cr-spinel is rich in Cr. Based on the olivine-melt equilibrium, the primary parental melt compositions are calculated. The high-Mg olivine-hosted picrite can be regarded as parental melt. Thus, the melting temperature and pressure are estimated: T=1600℃ and P=4.5 GPa. It suggests that the picrites are connected with the activity of mantle plumes. Their major element composition is comparable to many other CFBs by their high Fe8, (CaO/Al2O3)8 and low Na8, indicating a high pressure. All rocks display a similar chondrite-normalized REE patterns, i.e., enrichment of LREE, relative depletion of HFSE and absence of negative Nb and Ta but depletion in P and K. Some incompatible element ratios, such as La/Ta, La/ Sm, (La/Nb)PM, (Th/Ta)PM, are in a limited range, show that they were derived from the mantle plume, and there was no or little crustal contamination during magma ascent en route to the surface. They were generated by 7% partial melting of garnet peridotite. The axis of the plume might be located beneath Lijiang Town, Yunnan province.

Geochemical and Petrological Insights into the Neoproterozoic Moumba Metabasites: Implications for Crustal Processes in the West Congo Belt (Republic of Congo)

The West Congo Belt contains in its rocks of Neoproterozoic age from Nemba complex outcropping in the Moumba River. This West Congo belt is made up of a crustal segment of the Arcuaï-West Congo orogen which extends from southwest Gabon to the northeast of Angola. This study aims to constrain the geochemical signature Nemba complex of West Congo belt from the petrograhic and geochemical study on the whole rock. The petrographic data from this study show the Moumba metabasites are made up of amphibolites, metagabbros, epidotites and greenschists interstratified in the Eburnean metasediments and affected by mesozonal to epizonal metamorphism characterized by the retromorphosis of intermediate amphibolite facies minerals into greenschist facies. Whole-rock geochemical data indicate that these metabasites are continental flood basalts (CFB) of basic nature and transitional affinity emplaced in intraplate context. These continental flood basalts are generated from magma originating from a significantly enriched shallow mantle plume and this magma then contaminated by the continental crust during their ascent. The reconstruction of tectonic signature suggests that West Congo belt would result from closure of an ocean basin with subduction phenomena. This collision would be marked by the establishment of ophiolite complex. We show that this model is incompatible with the CFB nature of metabasites and the orogenic evolution of Neoproterozoic. It does not seem that we can evoke a genetic link with a subduction of oceanic crust, because the paleogeography of Neoproterozoic (Rodinia) is marked by intracontinental rifts linked to opening of Rodinia. We therefore suggest the non-existence of ophiolitic complex in western Congo belt and reject the collisional model published by certain authors. We confirm the currently available intracontinental orogen model.

Mantle plume,large igneous province and continental breakup——Additionally discussing the Cenozoic and Mesozoic mantle plume problems in East China

Based on the former workers' study results such as numerical simulation of fluid mechanics, seismic tomography of the whole earth and igneous rocks, the basie characteristics of mantle plumes are summarized in detail, namely the mantle plume, from the D" layer near the core-mantle boundary (CMB) of 2900 km deep, is characterized by the shape of large head and thin narrow conduit, by the physical property of high temperature and low viscosity. The LIP (large igneous province) is the best exhibition when the mantle plume ascends to the surface. According to the basie characteristics of the mantle plumes and the LIP, as well as the temporal-spatial relationships between the mantle plume and Continental breakup, the detailed research on petrology, geochemistry, temporal-spatial distribution, tectonic background of the Cenozoic-Mesozoic igneous rocks and gravity anomaly distribution in East China has been done. As a result, the Mesozoic igneous rocks in Southeast China should not be regarded as an example of typical LIP related to mantle plumes, for their related characteristics are not consistent with those of the typical LIPs related to mantle plumes. The Cenozoic igneous rocks in Northeast China have no the typical characteristics of mantle plumes and hotspots, so the Cenozoic volcanism in Northeast China might have no the direct relationships with the activity of mantle plumes.

Petrogenesis and Geochemistry of Neogene Basalts in Eastern Anhui

The basalt terrain of the Neogene Huangguoshan and. Guiwu Formations of eastern Anhui on the east side of the Tancheng-Lujiang fault belt is one of a few Cenozoic basalt terrains in eastern China for which detailed geochemical study has not been conducted. This paper reports the abundances of major elements and more than 20 trace elements (including REE) of 22 samples and the Nd, Sr and Pb isotopic compositions of 11 samples from the eastern Anhui basalt terrain, thus more or less systematically revealing the geochemical characteristics of this continental basalt suite. The paper discusses the origin of the basalt suite and the character and process of its mantle source. The basalt suite was derived from a heterogeneous continental lithospheric mantle with end members characteristic of the EMI-type oceanic basalt mantle, which was affected by mantle metasomatism (or enrichment of trace elements) and was characterized by a multi-stage evolution under open conditions.

Flood basalt hosted palaeosols:Potential palaeoclimatic indicators of global climate change

Since continental sediments （in addition to the marine geological record） offer important means of deciphering environmental changes, the sediments hosted by the successive flows of the continental flood basalt provinces of the world should be treasure houses in gathering the palaeoclimatic data. Palaeosols developed on top of basalt flows are potentially ideal for palaeoenvironmental reconstructions because it is easy to determine their protolith geochemistry and also they define a definite time interval. The present paper summarizes the nature of the basalt-hosted palaeosols formed on the flood basalts provinces from different parts of the ~lobe havin~ different ages.

Petrogenesis and Tectonic Implications of the Tertiary Choke Shield Basalt and Continental Flood Basalt from the Central Ethiopian Plateau

The voluminous Choke Shield basalts and flood basalts are distributed in the central Ethiopian Plateau.They are tholeiitic in composition and have OIB-like geochemical features.The ca.23 Ma Choke Shield basalts have SiO_(2)(47.1 wt.%-59.6 wt.%),MgO(1.01 wt.%-7.8 wt.%),Na_(2)O+K_(2)O(2.7 wt.%-8.4 wt.%),and display right inclined REE patterns((La/Yb)N=21.4-24.2) with enrichment of Nb,Ta,Zr,Hf and Pb in the primitive mantle-normalized trace element diagrams.They show low initial ^(87)Sr/^(86)Sr ratios(0.703 47-0.703 77) and high ε_(Nd)(t) values(+4.4 to+5.0).In comparison,the 24Ma high-Ti(HT1) flood basalts have SiO_(2)(38.9 wt.%-50.8 wt.%),MgO(3.9 wt.%-11.4 wt.%),Na_(2)O+K_(2)O(1.6 wt.%-5.8 wt.%), and display right inclined REE patterns((La/Yb)N=24-130.3) with enrichment of Nb,Ta,Zr,Hf,and Pb.They also show low initial ^(87)Sr/^(86)Sr ratios(0.703 30-0.704 44) and highε_(Nd)(t) values(+2.2 to+5.3).Both types of basalts were contaminated by minor crustal materials and underwent fractional crystallization of clinopyroxene,plagioclase,olivine,and minor Fe-Ti oxide.The Choke Shield basalts were generated by 1%-5% melting of garnet-spinel to phlogopite-bearing spinel lherzolite in a shallow zone of the mantle plume,while the flood basalts were formed by <20% melting of amphibole-bearing garnet to garnet-spinel lherzolite in a deeper zone of the same mantle plume.The mantle source beneath the central Ethiopian Plateau was significantly heterogeneous during the Tertiary.It was characterized by EMI and EMII end-members that were formed by the metasomatism of the different components.

Typical Oxygen Isotope Profile of Altered Oceanic Crust Recorded in Continental Intraplate Basalts

Recycled oceanic crust （ROC） has long been suggested to be a candidate introducing en- riched geochemical signatures into the mantle source of intraplate basalts. The different parts of oceanic crust are characterized by variable oxygen isotope compositions （δ^18O=3.7‰ to 13.6‰）. To trace the sig- natures of ROC in the mantle source of intraplate basalts, we measured the δ^18O values of clinopyroxene （cpx） phenocrysts in the Cenozoic basalts from the Shuangliao volcanic field, NE China using secondary ion mass spectrometer （SIMS）. The δ^18O values of the Shuangliao cpx phenocrysts in four basalts ranging from 4.10‰ to 6.73‰ （with average values 5.93‰±0.36‰, 5.95‰±0.30‰0, 5.58‰±=0.66‰e, and 4.55‰±= 0.38‰, respectively） apparently exceed those of normal mantle-derived cpx （5.6‰±0.2‰） and fall in the typical oxygen isotope range of altered oceanic crust. The δ^18O values display the negative correlations with the Eu, Sr anomalies of whole rocks and erupted ages, demonstrating that （1） the ROC is the main enriched component in the mantle source of the Shuangliao basalts and （2） the contributions of ROC var- ied with time. The basalt with the lowest δ^18O value is characterized by a significant K positive anomaly, highest H2O/Ce and Ba/Th ratios, suggesting that the mantle source of basalts with low δ^18O can also in- clude a water-rich sediment component that may be the trigger for partial melting. Considering the continuous subduction of the Pacific slab, the temporal heterogeneity of the source components is likely to be caused by the Pacific slab subduction.