PROVENANCE OF BASAL SEDIMENTS IN THE SOUTH CHINA SEA BASIN AT IODP EXPEDITION 367/368 SITE U1499

IODP Expeditions 367/368 drilled the northern margin of the South China Sea (SCS) basin to investigate its rifting history. Recovery of the material at, and just below, acoustic basement was important for evaluating a magma-poor or a magma-rich rifting model for the basin. Instead of recovering igneous or continental basement rock below the hypothesized acoustic basement seismic reflector, however, the base of Site U1499 contained sandstone with polymict clasts and matrix-supported breccia, overlying sedimentary cobbles.

Sixteen samples of fine to medium-grained sandstone cobbles from the base of Site U1499 were analyzed petrographically. The cobbles have an average framework composition of Q₁₈F₇₃L₉, with limited variability. Plagioclase and K-feldspar grains are extremely weathered and altered to kaolinite and sericite with many oxide inclusions. The degree of alteration varies among feldspar grains within a sample; some grains still show twinning, while others are almost unrecognizable as feldspars.

Monocrystalline quartz grains are subrounded to angular and show evidence of strain. Quartz grains contain abundant fractures, minimal overgrowth, and common inclusions of tourmaline, oxides, and needle-like minerals. Polycrystalline quartz (Qp) grains are subordinate (average Qp/Q ratio of 0.32) and vary from subangular to well rounded, which may indicate two sources: a first-cycle source proximal to Site U1499, and a source of recycled grains.

Lithic composition in the cobbles averages Lv₄₇Lm₁₉Ls₃₄, ranging 17-75% Lv, 0-60% Lm, and 0-83% Ls. Ls grains are mainly chert, with some sandstone and siltstone; Lm grains contain elongated quartz minerals and oriented mica minerals. In this study, Lv includes subequal abundances of volcanic and plutonic grains. Overall, the lithic assemblage indicates an arc and rifted continental margin provenance. For comparison, the plagioclase feldspar-rich composition of this unit suggests sources varying from continental block to dissected arc. As a result, petrographic analysis suggests that this unit was supplied from a relatively proximal source, potentially from granitoid basement rocks in southern mainland China. The cobble unit may have been deposited by a locally sourced gravity flow, based on its limited geographic distribution.