广东沿海地区基性岩脉地球化学及成因

Geochemistry and genesis of mafic dikes from the coastal areas of Guangdong Province, China.

广东沿海地区基性岩脉的成岩年龄是146～54 Ma,主要形成于白垩纪,次为侏罗纪和第三纪。可将工作区基性岩脉的形成时代分为5期:第一期146Ma,第二期138～132Ma,第三期112～105Ma,第四期99～82Ma,第五期75～54Ma。根据K_2O/Na_2O比值和K_2O含量,以及不相容元素分配模式的"Nb-Ta"异常,可以划分出两类基性岩脉。第一类K_2O/Na_2O比值<0.5和K_2O含量<1.3%,在微量元素MORB标准化图解中具有弱的"Nb-Ta"负异常。第二类K_2O/Na_2O比值>0.5和K_2O含量>1.3%,在微量元素MORB标准化图解中具有显著的"Nb-Ta"负异常。在K_2O-SiO_2图解中,第一类基性岩脉样品落入中钾岩石范围,第二类基性岩脉样品落入高钾岩石或钾玄岩范围。基性岩脉分布的重要特点是中钾基性岩脉分布于本区北部上地幔拗陷区,而高钾基性岩脉分布于本区南部地幔隆起区或斜坡区。两类基性岩脉有着不同的岩浆来源。在本区北部,软流圈物质入侵和熔蚀Ⅰ型富集地幔混合形成中钾基性岩脉,随着时代的由老至新和熔蚀作用的不断进行,岩浆中Ⅰ型富集地幔成分增加。晚期,岩浆活动向连平、新丰一带迁移。在本区南部,软流圈物质入侵并熔蚀Ⅱ型富集地幔,形成具有软流圈物质和Ⅱ型富集地幔混合特征的高钾基性岩脉。随着熔蚀作用的不断进行,岩浆中Ⅱ型富集地幔成分增加,晚期基性岩浆活动向三水裂谷、南澳裂谷迁移。

Whole rock K-Ar data yielded ages of 146 similar to 54 Ma for the mafic dikes from the coastal areas of Guangdong Province, Suggesting that emplacement of the mafic dikes took mainly place in the Cretaceous. The K-Ar data can be divided into five periods (146, 138 similar to 132, 112 similar to 105, 99 similar to 82, 75 similar to 54Ma). Two series of the mafic dikes (medium-K and high-K) can be distinguished according to K(2)O/Na(2)O ratios and K(2)O contents and the 'Ta-Nb' anomalies in the MORB-normalized trace element distribution patterns. The K(2)O/Na(2)O ratios of one series are < 0.5, and K(2)O contents are < 1.3% with weak negative 'Ta-Nb' anomalies. The K(2)O/Na(2)O ratios of another series are > 0.5, and K(2)O contents > 1.3% with strong negative 'Ta-Nb' anomalies. Using K(2)O vs. SiO(2) diagram, first series belongs to medium-K rocks, and another falls into the field of high-K rocks or shoshonitic rocks. The medium-K mafic dikes are distributed in upper mantle downwarping areas and the high-K mafic dikes are distributed in the upper mantle uplifting areas or slope areas. Two series of the mafic dikes have different magmatic source. In the north part of the area, the asthenospheric material invaded and corroded the lithospheric enriched mantle I, forming the medium-K mafic dikes. With the ages becoming younger, the contribution of lithospheric enriched mantle I to mafic dike magmas was increased. Late, the mafic magmatism in the north migrated toward Liangping, Xinferig. In the south, the asthenospheric material invaded and corroded the lithospheric enriched mantle II, forming the high-K mafic dikes. With the ages becoming younger, the contribution of lithospheric enriched mantle II to mafic dikes was increased, showing the lithospheric mantle in the south was increasingly corroded by the asthenospheric material. I-ate, the mafic magmatism in the South migrated to Shanshui rift and Nanao rift.