Paleo-Pacific subduction-accretion: Evidence from Geochemical and U-Pb zircon dating of the Nadanhada accretionary complex, NE China
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The Nadanhada Terrane, located along the eastern margin of Eurasia, contains a typical accretionary complex related to paleo-Pacific plate subduction-accretion. The Yuejinshan Complex is the first stage accretion complex that consists of meta-clastic rocks and metamafic-ultramafic rocks, whereas the Raohe Complex forms the main parts of the terrane and consists of limestone, bedded chert, and mafic-ultramafic rocks embedded as olistolith blocks in a weakly sheared matrix of clastic meta-sedimentary rocks. Geochemical data indicate that the Yuejinshan metabasalts have normal mid-ocean ridge basalt (N-MORB) affinity, whereas the Raohe basaltic pillow lavas have an affinity to ocean island basalts (OIB). Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon analyses of gabbro in the Raohe Complex yield a weighted mean 206Pb/238U zircon age of 216±5Ma, whereas two samples of granite intruded into the complex yield weighted mean 206Pb/238U zircon ages of 128±2 and 129±2Ma. Laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Pb zircon analyses of basaltic pillow lava in the Raohe Complex define a weighted mean age of 167±1Ma. Two sandstone samples in the Raohe Complex record younger concordant zircon weighted mean ages of 167±17 and 137±3Ma. These new data support the view that accretion of the Raohe Complex was between 170 and 137Ma, and that final emplacement of the Raohe Complex took place at 137-130Ma. The accretion of the Yuejinshan Complex probably occurred between the 210 and 180Ma, suggesting that paleo-Pacific plate subduction was initiated in the Late Triassic to Early Jurassic.
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