EARLY MESOZOIC, POST-COLLISIONAL SHOSHONITIC LAMPROPHYRES ALONG THE WESTERN MARGIN OF THE SOUTH CHINA OROGEN: GEOCHEMICAL CHARACTERISTICS AND TECTONIC IMPLICATIONS

Abstract

The Anhua-Xupu shoshonitic lamprophyres, emplaced along the western margin of the South China orogen (Cathaysian block), formed about 207 Ma. They are characterized by high K2O/Na2O ratios (0.84-1.72), high K2O + Na2O contents (>4.33-6.41%), low TiO2 contents (0.54-1.21%), variable but high Al2O3, contents (13.32-16.28%), strong enrichments in LILE and LREE, and distinct depletions in HFSE (such as Nb, Ta, and Ti) with no Eu anomalies. The rocks possess highly initial radiogenic 87Sr/86Sr ratios (0.721565-0.722292) and nonradiogenic 143Nd/144Nd ratios (0.511836-0.511869), corresponding to εNd(t) = -9.81 to -10.45. Such geochemical characteristics are comparable to the Åva lamprophyres in southern Finland and Russian Karelia, the melilite-bearing rocks from the Montefiascone Volcanic Complex (Roman Magmatic Province), and the Damavand shoshonitic volcanics (Central Alborz, northern Iran). Geochemical and isotopic signatures shown by the Anhua-Xupu primary magmas require a clinopyroxene- and phlogopite-rich mantle source, whereas partial melting of a veined lithospheric mantle accounts for the occurrence of different primary magmas characterized by relatively constant Sr- and Nd-isotopic compositions. Ages of the depleted mantle Nd-model (1.30-1.37 Ga) implies that mantle enrichment may have been related to Proterozoic suturing of the Yangtze and Cathaysian blocks. Considering the regional tectonic evolution, we suggest that the generation of these shoshonitic lamprophyres was a product of post-collisional intralithospheric extension during Early Mesozoic time. Copyright © 2006 by V. H. Winston & Son, Inc. All rights reserved.