Chronology and Geochemistry of Mesozoic Volcanic Rocks in the Linjiang Area,Jilin Province and their Tectonic Implications

Zircon U-Pb ages and geochemical analytical results are presented for the volcanic rocks of the Naozhigou, Ergulazi, and Sidaogou Formations in the Linjiang area, southeastern Jilin Province to constrain the nature of magma source and their tectonic settings. The Naozhigou Formation is composed mainly of andesite and rhyolite and its weighted mean ^206pb/^238U age for 13 zircon grains is 222±1 Ma. The Ergulazi Formation consists of basaltic andesite, basaltic trachyandesite, and andesite, and six grains give a weighted mean ^206pb/^238U age of 131±4 Ma. The Sidaogou Formation consists mainly of trachyandesite and rhyolite, and six zircon grains yield a weighted mean ^206pb/^238U age of 113±4 Ma. The volcanic rocks have SIO2=60.24%-77.46%, MGO=0.36%-1.29% （Mg#=0.32-0.40） for the Naozhigou Formation, SIO2=51.60%-59.32%, MGO=3.70%-5.54% （Mg#=0.50-0.60） for the Ergulazi Formation, and SIO2=58.28%-76.32%, MGO=0.07%-1.20% （Mg#=0.14-0.46） for the Sidaogou Formation. The trace element analytical results indicate that these volcanic rocks are characterized by enrichment in light rare earth elements （LREEs） and large ion lithophile elements （LILEs）, relative depletion in heavy rare earth elements （HREEs） and high field strength elements （HFSEs, Nb, Ta, and Ti）, and negative Eu anomalies. Compared with the primitive mantle, the Mesozoic volcanic rocks in the Linjiang area have relatively high initial ^87Sr/^86Sr ratios （0.7053-0.7083） and low εNd（t） values （-8.38 to -2.43）, and display an EMⅡ trend. The late Triassic magma for the Naozhigou Formation could be derived from partial melting of a newly accretional crust with the minor involvement of the North China Craton basement and formed under an extensional environment after the collision of the Yangtze Craton and the North China Craton. The Early Cretaceous volcanic rocks for the Ergulazi and Sidaogou Formations could be formed under the tectonic setting of an active continental margin related to the westward subduction of the Izanagi plate.

Comparison of Redox States between the Ultramafic Bodies of Xigaze and Luobusha Ophiolites, Tibet, China

The tectonic setting of podiform chromitite formation still remains highly debated. There is a close correlation between tectonic settings and oxygen fugacity(fO2)(e.g., Ballhaus, 1993;Dare et al., 2009;Parkinson and Arculus, 1999). Here we present results of fO2 of chromites determined by M?ssbauer spectroscopy from both the Luobusha and Dazhuqu areas along Yarlung Zangbo suture zone, Southern Tibet. The fO2 values(-1.02~0.04 log units against the FMQ buffer) and Cr#(22~54%) in chromites from lherzolites and harzburgites of both areas are similar to those of abyssal peridotites, indicating that they may be residues after partial melting at spreading centers. However, both dunite envelopes and chromitites from Luobusha have high fO2 values(0.04~2.25 log units) and Cr#(73~84%), showing an affinity to boninitic melts, and thus form in a suprasubduction zone. Dazhuqu dunites show diverse fO2 values(-0.22~2.19 log units) and Cr#(22~82%), indicating that they form in distinct settings. Chromitites and chromite dunites from Dazhuqu have low fO2 values(-0.3~0.71 log units) and Cr#(16~63%), both of which are similar to those of MORB-like basalts, inferring that they form in an extensional setting. Both high-Cr and high-Al chromitites from other typical podiform chromite ore deposits, such as Kempirsai, Oman, and Albania ophiolites, also show high fO2 values(e.g., Chashchukhin and Votyakov, 2009;Melcher et al., 1997;Quintiliani et al., 2006;Rollinson and Adetunji, 2015), while the distribution-limited small chromitites and chromite dunites from Dazhuqu exhibit low fO2 values. The phenomenon infers that the suprasubduction zone is more beneficial to the formation of podiform chromitites.