Petrogenesis of the Langdu High-K Calc-Alkaline Intrusions in Yunnan Province: Constraints from Geochemistry and Sr-Nd Isotopes

The Langdu high-K calc-alkaline intrusions are located in the Zhongdian area, which is the southern part of the Yidun island arc. These intrusive rocks consist mainly of monzonite porphyry, granodiorite, and diorite porphyry. The K20 content of majority of these rocks is greater than 3%, and, in the K20-SiO2 diagram, all the samples fall into the high-K calc-alkaline to shoshonitic fields. They are enriched in light rare earth elements （LREEs） and depleted in heavy rare earth elements （HREEs; LaN/YbN = 14.3-21.2）, and show slightly negative Eu anomalies （6Eu = 0.77-1.00）. These rocks have high K, Rb, Sr, and Ba contents; moderate to high enrichment of compatible elements （Cr = 36.7-79.9 ppm, Co = 9.6-16.4 ppm, and MgO = 2.2%-3.4%）; low Nb, Ta, and Ti contents, and characteristic of low high field strength elements（HFSEs） versus incompatible elements ratios （Nb/Th = 0.75, Nb/La = 0.34） and incompatible elements ratios （Nb/U = 3.0 and Ce/Pb = 5.1, Ba/Rb = 12.0）. These rocks exhibit restricted Sr and Nd isotopic compositions, with （87Sr/S6Sr）i values ranging from 0.7044 to 0.7069 and ENd（t） values from -2.8 to -2.2. The Sr-Nd isotope systematic and specific trace element ratios suggest that Langdu high-K calc-alkaline intrusive rocks derived from a metasomatized mantle source. The unique geochemical feature of intrusive rocks can be modeled successfully using different members of a slightly enriched mantle, a slab-derived fluid, and terrigenous sediments. It can be inferred that the degree of partial melting and the presence of specific components are temporally related to the tectonic evolution of the Zhongdian island arc. Formation of these rocks can be explained by the various degrees of melting within an ascending region of the slightly enriched mantle, triggered by the subduction of the Garz^--Litang ocean, and an interaction between the slab-derived fluid and the terrigenous sediments.

Cenozoic Adakite-type Volcanic Rocks in Qiangtang,Tibet and Its Significance

Volcanic rocks in the study area, including dacite, trachyandesite and mugearite, belong to the intermediate-acid, high-K calc-alkaline series, and possess the characteristics of adakite. The geochemistry of the rocks shows that the rocks are characterized by SiO2>59%, enrichment in A12O3(15.09-15.64%) and Na2O (>3.6%), high Sr (649-885 μg/g) and Sc, low Y contents (<17 μg/g), depletion in HREE (Yb<1.22 μg/g), (La/Yb)N>25, Sr/Y>40, MgO<3% (Mg<0.35), weak Eu anomaly (Eu/Eu=0.84-0.94), and lack of the high field strength elements (HFSE) (Nb, Ta, Ti, etc.). The Nd and Sr isotope data (87Sr/86Sr=0.7062-0.7079, 143Nd/144Nd=0.51166-0.51253, εNd= -18.61-0.02), show that the magma resulted from partial melting (10%-40%) of newly underplated basaltic lower crust under high pressure (1-4 GPa), and the petrogenesis is obviously affected by the crust's assimilation and fractional crystallization (AFC). This research will give an insight into the uplift mechanism of the Tibetan plateau.

Ordovician Granitoids and Silurian Mafic Dikes in the Western Kunlun Orogen, Northwest China:Implications for Evolution of the Proto-Tethys

The western Kunlun orogen in the northwest Tibet Plateau is related to subduction and collision of Proto-and Paleo-Tethys from early Paleozoic to early Mesozoic. This paper presents new LA-ICPMS zircon U-Pb ages and Lu-Hf isotopes, whole-rock major and trace elements, and Sr–Nd isotopes of two Ordovician granitoid plutons(466–455 Ma) and their Silurian mafic dikes(～436 Ma) in the western Kunlun orogen. These granitoids show peraluminous high-K calcalkaline characteristics, with(^(87)Sr/^(86)Sr)_i value of 0.7129–0.7224, ε_(Nd)(t) values of -9.3 to -7.0 and zircon ε_(Hf)(t) values of -17.3 to -0.2, indicating that they were formed by partial melting of ancient lower-crust(metaigneous rocks mixed with metasedimentary rocks) with some mantle materials in response to subduction of the Proto-Tethyan Ocean and following collision. The Silurian mafic dikes were considered to have been derived from a low degree of partial melting of primary mafic magma. These mafic dikes show initial ^(87)Sr/^(86)Sr ratios of 0.7101–0.7152 and ε_(Nd)(t) values of -3.8 to -3.4 and zircon ε_(Hf)(t) values of -8.8 to -4.9, indicating that they were derived from enriched mantle in response to post-collisional slab break-off. Combined with regional geology, our new data provide valuable insight into late evolution of the Proto-Tethys.

Petrogenesis of Granites from the Ediacaran Socorro Batholith,SE Brazil:Constraints from Zircon Dating,Geochemistry and Sr-Nd-Hf Isotopes

Whole rock elemental and Sr-Nd isotope geochemistry and in situ zircon Hf isotope geochemistry were used to identify the sources of the Neoproterozoic granites from the Socorro batholith,Socorro-GuaxupéNappe(SGN),South Brasilia Orogen,Brazil.Zircon trace elements and Hf isotope geochemistry provided information about sources and crystallization(T,f_(O2))conditions.Three main types of granites built the bulk of the batholiths,beginning with probably pre-collisional~640–630 Ma charnockites,and ending with~610 Ma voluminous post-collisional high-K calc-alkaline(HKCA)Ⅰ-type granites(Bragan?a Paulista-type).Several types of leucogranites were generated from 625 to 610 Ma,spanning the interval from collisional to post-collisional tectonics.Two charnockite bodies occur in the study area:the~640 Ma Socorro charnockite has remarkable chemical similarities with Braganca Paulista-type granites,but higherε_(Nd)(t)=-6.1 and average zirconε_(Hf)(t)=-9.1 and lower^(86)Sr/^(87)Sr_(t)(0.7093)values,indicative of more juvenile and water-poor source.The~633 Ma Atibaia charnockite has distinct geochemical signature(lower Mg^(#)and Sr content;higher Zr),more negativeε_(Nd)(t)=-14.1,similar average zirconε_(Hf)(t)=-8.9,and much higher^(86)Sr/^(87)Sr_(t)=0.7197,probably reflecting a larger component from old crust.The predominant~610 Ma Bragan?a Paulista-type granites were emplaced in a post-collisional setting,and correspond to porphyritic biotite-hornblende monzogranites of high-K calc-alkaline character with 61 wt.%–67 wt.%SiO_(2),high Mg^(#)(39–42),Sr/Y(19–40),La/Yb(12–69),highly negativeε_(Nd)(t)(-12.3 to-12.9)and zirconε_(Hf)(t)(-12 to-17)and^(87)Sr/^(86)Sr_(t)=0.7119–0.7131.These features are interpreted as indicative of magma generation in a thickened crust,where melts from enriched mantle sources emplaced in the lowermost crust,heated host old continental crust rocks(gneisses and granulites)and partially mixed with their melting products.Leucogranite plutons(SiO_(2)>72 wt.%)occurring in the southern portion of the batholith have a range of geochemical and isotope signatures,reflecting melting of crustal sources in space and time between~625 Ma(Bocaina Pluton)and~610 Ma(Bairro da Pedreira Pluton).Highly negativeε_(Nd)(t)(-16.2)and average zirconε_(Hf)(t)=-16,and high^(87)Sr/^(86)Sr_(t)(0.7156–0.7171)are consistent with relatively old orthoand paragneiss sources similar to those which generated regionally abundant migmatites and anatectic granites in the collisional to post-collisional setting.