CHARACTERISTICS AND ORIGIN OF HIGH-NA ADAKITE-LIKE ROCKS: THE LATE CRETACEOUS ZHANTANG ANDESITES IN THE HUICHANG BASIN, JIANGXI PROVINCE
CAI Zhiyong~(1,2), XIONG Xiaolin~1 and SUN Sancai~3 (1.Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, GD 510640, China;2.Northwestern Hubei Survey of Geology and Mineral Resources, Xiangfan, HB 441003, China)
The Zhantang andesites occur in the Cretaceous Huichang Basin, Eastern Nanling Range of SE China, and were formed under an intracontinental, extensional tectonic setting that was not associated with subduction. They are high-Na (up to 8.46% Na_2O), high-Al trondhjemitic rocks, with high concentrations of Sr and Ba, low concentrations of MgO, CaO, Cr, Co Ni, Y and HREE, and high Sr/Y and La/Yb and low Rb/Sr ratios. Partial melting of mantle lithosphere, magmatic mixing or fractional crystallization cannot explain the origin of these compositionally distinctive magmas, while partial melting of garnet-containing basaltic protolith such as subducted slab or basaltic underplate can account for many features of these magmas. The high Na, Al and high Sr/Y and La/Yb features of the Zhantang andesites are similar to those of adakite resulted from slab-derived melting. However, the Zhantang andesites were not derived from subducted slab. As compared with adakite, they are much higher in Na_2O but lower in CaO. The ε_(Nd)(t) (-2.3～-3.8) and initial ()~(87)Sr/~(86)Sr (0.707～0.708) are also significantly different from those of most adakites which have like-MORB isotopic compositions. These chemical and isotopic data together with the geological setting suggest that subducted slab was not involved in producing these magmas. Alternative derivation of these magmas is therefore the partial melting of underplated basaltic lower crust. Partial melt composition of basalt is closely related to source composition. The high Na_2O and low CaO signature for the Zhantang magmas suggests a compositionally particular source. Comparison shows that the Zhantang andesites are generally compositionally similar to the high-pressure partial melts of the alkali-rich, high Na_2O and low CaO basalt (Rapp and Watson, 1995). We therefore suggest that the Zhantang high-Na and low-Ca adakite-like magmas were produced, at a pressure high enough to stabilize garnet (≥10×~8 Pa), by melting of an alkali-rich basaltic (amphibolitic) underplate. The melting probably took place in response to the continued magma underplating and thickening at the base of the crust of the Eastern Nanling Range.