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The tectonic setting of Jiangda-Weixi magmatic belt is still controversy. Two different interpretations include the eastward subduction of Longmucuo-Shuanghu-Changning-Menglian Paleo-Tethys main ocean basin, and the westward subduction and subsequent collision of Jinshajiang Paleo-Tethys main ocean basin. These controversies concern different models for tectonic evolution of Paleo-Tethys. A new set of intrusive complex has been discovered in Pantiange area, the southern part of Jiangda-Weixi magmatic belt. The main components include dioritic, granodioritic and granitic rocks, which have been mylonitized. The zircons from granitic rocks have U-Pb mean age of (243.0±4.8) Ma. The age is considered as crystallization age of granitic rocks. Dioritic rocks have high contents of Na2O and Al2O3, and moderate content of K2O, suggesting they belong to metaluminous and calc-alkaline series. Granodioritic rocks has high contents of K2O and Al2O3, but low content of Na2O, suggesting they belong to peraluminous and shoshonite, high potassium calc-alkaline series. Granitic rocks have moderate content of Na2O and low content of K2O, and belong to metaluminous-peraluminous and low-potassium series. All three types of rocks are enriched in LILEs (such as Rb, Th, U), depleted in HFSEs (such as Nb, Ta, P, Ti) and LILEs (such as Ba, Sr), with high values of LREE/HREE ratio. The dioritic rocks have no obvious Eu anomaly, whereas granodioritic and granitic rocks show weak negative Eu anomaly. The granitic rocks have zircon εHf(t) values of-11.92--8.54, whole rocks initial 87Sr/86Sr ratio of 0.717 229-0.725 994, and εNd(t) values of-14.2--7.8. These characteristics show that dioritic magmatism is formed by partial melting of Paleoproterozoic basaltic lower crust; granodioritic magmatism is formed by partial melting of metamorphic mudstones of Paleoproterozoic upper crust with the addition of mantle material; granitic magmatism originates from partial melting of metamorphic graywacke in Paleoproterozoic upper crust, and then underground crystallization fractionation with the addition of mantle material. Combined with regional tectonic setting and petrogeochemical characteristics, it is proposed that Jiangda-Weixi magmatic belt is formed by the eastwards subduction of Paleo-Tethys. [ABSTRACT FROM AUTHOR]

Gejiu area in Yunnan is located at a faulted depression basin in the southwestern of Youjiang continental margin rift basin, which is the transitional zone between Yangtze platform and South China Caledonian geosyncline, and the tectonic location is in the southwestern South China. The geology, geochemistry of Late Ladinian-Norian meta-basalt in Tabai area of Gejiu, Yunnan indicate that these basalts have undergone strong metamorphism because of regional dynamic tectonics. The meta-basalts are characterized by low contents of silica, calcium, magnesium, and enrichment of incompatible elements(Ti, P, K, etc.). They can be classified as high-Ti alkaline basalts. The trace elements are characterized by extremely low contents of Rb and Ba, severely depletion in Sr and enrichment in Ta, Ti, Zr, Hf, Th, which is similar to the weak crustal contamination of the Emeishan high-Ti basalt. The chondrite-normalized REE pattern shows LREE enrichment and weak positive Eu anomaly. The Sr-Nd isotopes suggest that meta-basalts in Tabai area of Gejiu are mainly from the primitive mantle and fall in the oceanic island basalts(OIB), which are of the same origin as Emeishan high-Ti basalts. Emeishan mantle plume activity has ended in Late Permian. Therefore, the stretching of Yangtze massif in Triassic and the subduction of Simao massif to Yangtze massif induce Emeishan mantle plume stigma to a low degree of partial melting. Because the influence of mantle plume activity lasts for a long time, the energy of mantle plume activity is low in some areas, the basaltic magma does not erupt to the surface and forms magma chamber. Meta-basalts in Tabai area of Gejiu are formed in the relatively weak stage of the center of mantle plume magma activity, especially in the edge zone of mantle plume activity center, the relatively thick lithosphere, and low melting degree. The magma chamber activity in Late Triassic re-erupts and forms basalt. [ABSTRACT FROM AUTHOR]

Gubaoquan dolerite dykes are predominantly intruded into Ordovician-Devonian granodiorites, and a few are interspersed in Early Paleozoic strata. The dykes are divided into two different rock types, including dolerite and sillite. The mainly minerals are plagioclase and pyroxene, with minor am phi bole, biotite and some accessory minerals. The results of zircon UPb dating yield an age of (285. 0 ± 4. 7) Ma, indicating the emplacement time of dolerite dykes, which is coeval to the formation age of dolerite dykes in other places of Beishan area. The εNd (t) of Gubaoquan dolerite varies from 6. 39 to 6. 78, indicating that the parental magma is derived from a depleted mantle source. The whole rock Sr-Nd isotopic simulation suggests that the magma undergoes a 10 %-15 % of contamination by upper crust materials during emplacement process. The LREE/HREE ratio of dolerite varies from 1. 72 to 2. 17, and the chondrite normalized REE pattern displays a flat pattern. The primitive mantle-normalized trace element spider diagram shows obvious negative anomalies of Nb-Ta. The characteristics of rare earth and trace elements are different from Bachu dolerite associated with mantle plume activities in Tarim block, but are similar with Yin'aoxia (east part) and Podong (west part) dolerites in Beishan area. Integrated with the existing research results, it is suggested that Gubaoquan dolerite dykes are formed after the closure of Paleo-Asian ocean, as the result of basaltic magmatism caused by the decompression and partial melting of asthenosphere due to lithosphere delamination and break-off in an extensional environment. [ABSTRACT FROM AUTHOR]

Laocheng pluton exposed in Ningshan area of South Qinling, together with Wulong pluton. consist of an important part of Early Mesozoic granitoid magmatism in central South Qinling. Abundant mafic enclaves can be found in different units of Laocheng pluton. Analysis results of zircon U-Pb ages show that the host rock from Laocheng pluton is emplaced in 213-219 Ma, and its mafic enclaves are crystallized in 211 Ma, which are more or less simultaneous with the formation of Wulong pluton. Geochemical data show that the host rock from Laocheng pluton is granodiorite and quartz monzonite in composition with the characteristics of low Si. rich alkali, weak peraluminous and high Mg# (52-67), and belongs to high-K calc-alkaline I-type series; mafic enclaves are mainly dioritic, having the characteristics of low Si02 (52. 6%-62. 4%), rich alkali and high Mg# (63-72), belonging to shoshonite series. Compared with the host rock, the mafic enclaves have lower initial N(87 Sr)/N(86 Sr) (0.704 66-0.705 82), high ɛNd(t) (-4.3- -1.6) and higher Mg# , suggesting that the primary magma of the enclaves might have originated from a basaltic magma. Sr-Nd isotope geochemistry suggests that the host rock and mafic enclaves have exchanged prior to the formation of Laocheng pluton, and an equilibrium has been reached. Compared with Sr-Nd isotope ratio of basement rocks exposed in South Qinling, it is suggested that the magma of Laocheng granite originates from Neoproterozoic mafic crustal material. Relationship between host rock and mafic enclaves indicates that a magma mixing between the crystallizing acid magma and the basic magma slightly later injects into the magma chamber; these two magmas are not completely miscible during the mixing process. [ABSTRACT FROM AUTHOR]