Sr- Nd- Pb ISOTOPIC GEOCHEMISTRY OF LATE-PALEOZOIC MAFIC VOLCANIC ROCKS IN THE SURROUNDING AREAS OF THE GONGHE BASIN, QINGHAI PROVINCE AND GEOLOGICAL IMPLICATIONS

Abstract

The late-Paleozoic mafic volcanics in the surrounding areas of the Gonghe basin are distributed in the Aneymaqen ophiolite zone, Zongwulong tectonic zone and Kuhai-Saishitang tectonic zone, respectively. N-MORB in the Aneymaqen ophiolite zone have higher 87Sr /86Sr(350Ma) ratios (0.7066 - 0.7084), highεNd(350Ma) ratios (12.2 - 12.8) and low 206Pb/204Pb initial ratios (17.72 - 17.79). These isotopic ratios are similar to those of the same type rocks in the Mianlue ophiolite zone and the low 206Pb/204Pb and high 143Nd/144Nd N-MORB in Indian Ocean. The Sr and Nd isotopic rations in OIB cropped out in the same zone can be compared with the hotspot OIB in the Indian Ocean but 206Pb/204Pb ratios are lower, similar to the low 143 Nd/144 Nd MORB in the ocean. N-MORB in the Zongwulong tectonic zone have 87Sr /86Sr(350Ma) ratios = 0. 7041 - 0. 7058, εNd(350Ma) = 6.1 - 8.4 and 206Pb/204Pb = 17.51 - 17.90, and belong to the high 143Nd/144Nd N-MORB. The continental rift basalts in the Kuhai-Saishitang zone show high 87 Sr/86 Sr ratios (0.7115 0.7104) and lowε Nd(350Ma) (-1.7 -2.5), and have little difference from the above MORB in 206Pb/204Pb ratios (17.64 17.46). The above isotopic characteristics imply tectonic settings and contamination degree from continental crust for the mafic rocks. The Aneymaqen N-MORB are products generated in the mid-ocean ridge. The isotopic similarity to the same type rocks in the Mianlue zone indicates that they could have come from the common source region, while the Aneymaqen OIB represent materials derived from the hotspot. The Zongwulong MORB similarly belong to the depleted mantle-originated products and indicate that the zone once rifted to an ocean. The high Sr and low Nd isotopes of the mafic orcks in the Kuhai-Siashitan zone reflect the strong continental crust imprint, relating to their eruption on the Precambrian basement. Combined with the low Nb/U and Ce/Pb ratios from the both MORB and OIB, it can be inferred that the rocks could be derived from a source mixed between EMII and DMM materials through metasometism. Conventionally, obvious Dupal anomaly (Δ 208Pb/204 Pb = 46 - 103 and Δ 207Pb/204Pb = 4 - 18; 87Sr/86Sr > 0.704 for most samples) can mean that the MORB and OIB are remnants of oceanic crust spatially formed in the site of the present Indian Ocean. However, the more and more disicoveries of the Dupal anomaly in the North hemisphere could be interpreted by "focused subduction" in an archipelagic setting like today's SE Asia.