Abstract Geochronology of continental flood basalts sampled from the Emei large igneous province (LIP) on the western margin of the Yangtze platform was investigated by the laser microprobe 40 Ar/ 39 Ar dating technique. These basalts yield a fairly wide range of 40 Ar/ 39 Ar ages, varying from 259 to 135 Ma. One basalt sample, at least altered, recorded the oldest 40 Ar/ 39 Ar age of about 259 Ma, corresponding to a peak eruption age of the Emei LIP continental flood basalts. Most of the samples yield much younger ages from 135 to 177 Ma, which are consistent with the K‐Ar ages for the same samples (122.8–172.1 Ma). The dating data suggest that these Permian basalts had been widely affected by the regional tectonothermal event at 177–135 Ma. The event was probably caused by the convergence and collision among the Laurasia, Yangtze and Qiangtang‐Qamdo continental blocks on the eastern margin of the Qinghai‐Tibet plateau after the late Triassic. The age of the event reflects the timing of the peak collisional orogeny.
In order to document the chemical characteristics of sources of a continental flood basalt (CFB) and to understand its origin, we studied four typical picrite-basalt flows and three picritic intrusions in the Emeishan picrite-cluster field (5 x 104 km2 area), located in the southwestern part of the Emei large igneous province. Major and trace elements, and Sr-Nd-Pb isotopic compositions, are reported for picrites and associated basalts in the picrite-cluster field, regarded as the axial area of the Emei mantle plume. These rocks exhibit a range of MgO contents (4.2-27.2%), variable degrees of LREE enrichment (LaN/YbN = 2.7-29.0), and heterogeneous isotopic ratios (∊Nd(t) = 4.84 to -4.00; initial 87Sr/86Sr = 0.7023-0.7064; 206Pb/204Pb = 18.034-20.641). Nd, Sr, and Pb isotopic covariations are similar to at least three groups of the picrites and basalts. Ontong Java-like lavas yielded low (87Sr/86Sr)i (<0.7045), high ∊ Nd(t) (>2.5), and the lowest (Ce/Yb)N (∼4), La/Sm (∼3), Th/Nb (∼0.75), La/Nb (∼0.85), and Ba/Nb (∼8) ratios, suggesting a focus zone (FOZO)-like mantle source. Cook-Austral-like lavas have high 206Pb/204Pb (>19.0), low (87Sr/86Sr)i (<0.7045), and high Sm/Yb (>4), Nb/Y (>1) and Zr/Y (>8), similar to the HIMU component. Heard-like lavas yielded high (87Sr/86Sr)i (0.7044 ∼ 0.7060), low ∊Nd(t) (1.7 to -3.0), 206Pb/204Pb (18.75-18.03) and 208Pb/204Pb (37.8-39.0), and high (Ce/Yb)N (>10), Ba/Nb (>30), and Th/Nb (>0.1), which might be explained by mixing of picritic magmas from the plume with lamproitic melt from continental lithospheric mantle. Three samples outside the three groups have Sr-Nd isotopic signatures similar to EM I, and high Th/Nb (>0.2), La/Nb (>1.5), and low Nb/U (<25), suggesting a pelagic sediment source component. The picritic intrusives have low ∊Nd(t) (0.32-0.59), intermediate (87Sr/86Sr)i (0.7037-0.7042), very high Th/Nb (0.11-0.20), and low Nb/U (<25), implying minor crustal assimilation. The trace elements and Sr-Nd-Pb isotope geochemistry of the Emei picrites and associated basalts reflect processes involving: (1) partial melting of a heterogeneous, eclogite-bearing source to produce picritic magmas; then (2) mixing with small amounts of melt from the continental lithospheric mantle; and (3) crustal assimilation during picritic magma ascent. The heterogeneity of this source is related to recycling of subducted oceanic crust and associated sediments into the deep upper mantle.
Abstract 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 low-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.
Abstract: The Tongkuangyu metaporphyry copper deposit formed in the Early Proterozoic is tectonically located in the northern part of the Zhongtiao Λ‐type palaeorift depression, on the southern margin of the North China platform. The genesis of the deposit has long been a hot subject of research of geologists working on mineral deposits. By means of the protolith reconstruction and fluid inclusion and isotope studies of host rocks of the deposits, the authors hold that this deposit is the product of combined exogenic, endogenic and metamorphic processes. Its formation mainly involved the following four stages: (1) the stage of formation of Mg‐ and Fe‐rich tuffaceous variegated cupriferous sand‐shale and K‐rich biomodal volcanic rocks; (2) the early stage of formation of the metaporphyry copper deposit; (3) the stage of formation of the metaporphyry copper deposit; and (4) the stage of reworking of the metaporphyry copper deposit.
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