浙江浦江晚中生代埃达克质火山岩的成因

Petrogenesis of late Mesozoic adakitic andesites from Pujiang area in Zhejiang Province,eastern China.

浙江浦江地区白垩纪劳村组存在一套独特的中酸性火山岩,其主量元素表现出富 Al_2O_3(>16%)和 Na_2O(4.01%～4.52%),高 Na_2O/K_2O(1.94～5.51,平均2.89),高 Sr(583～643μg/g),贫 Y(11.4～13.2μg/g)和 HREE(如 Yb=1.16～1.26μg/g),以及低(^(87)Sr/^(86)Sr)_i(0.7041～0.7043)和高ε_(Nd)(t)(0.3～2.3)。这些火山岩在元素-同位素组成上明显不同于浦江地区或区域上的晚中生代中酸性火山岩,而与现代俯冲带产出的埃达克岩类似。由于这些岩石相对于晚中生代的基性岩浆具有更低的(^(87)Sr/^(86)Sr)_i 比值,因此难以通过玄武岩浆的分离结晶或同化混染作用来解释,最有可能是地壳部分熔融的产物,在构造背景方面,目前还没有确切的证据说明区域存在晚中生代洋壳的俯冲作用,我们倾向于认为劳村组埃达克质火山岩可能为新增生玄武质地壳部分熔融的结果,其形成与区域上中生代地壳增生作用有直接的动力学联系。

Late Mesozoic adakitic andesites found in the Laocun Formation from the Pujiang area in Zhejiang Province are geochemically distinct from the contemporaneous calc-alkaline felsic volcanic rocks in the southeastern coast areas of China. These rocks span a SiO2range of 57.64% - 62.61% and MgO 2.29% - 3.43% , characterized by high Al2O3 （ 〉 16% ）, Na2O （4.01% - 4.52%）, Na2O/K2O （1.94-5.51 with an average of 2.89） and Sr （583-643 μg/g）, and low Y （11.4-13.2 μg/g） and HREE （e. g. , Yb = 1.16 - 1.26 μg/g）. They have lower （^87Sr/^86Sr）i（0. 7041 -0.7043） and higher εNd（t） （0.3 --2.3） than the contemporaneous counterparts in the area and along the southeastern coast regions of China. All these geochemical features are similar to adakites occurring in modern subduction zones. Their even lower （^87 Sr/^86 Sr）i ratios than the contemporaneous mafic rocks are inconsistent with an origin of differentiates of the mafic magmas by fractional crystallization or assimilation via fractional crystallization. Since there lacks robust evidence for late Mesozoic oceanic ridge subduction toward the southeastern coast regions of China, these rocks were probably derived from crustal anatexis, and the higher εNd （ t ） and lower TDM called for an important contribution of juvenile material in the source. Considering the late Mesozoic extensional regime in the region, we relate the petrogenesis of these adakitic magmas to the Mesozoic crustal growth, consistent with the existence of Mesozoic high-εNd （t） and low-TDM＂ Shi-Hang belt＂.