粤西阳春中生代钾玄质侵入岩及其构造意义: Ⅰ.岩石学和同位素地质年代学

Mesozoic shoshonitic intrusives in the Yangchun Basin, western Guangdong, and their tectonic significance:Ⅰ . Petrology and isotope geochronology

粤西阳春地区马山二长岩的岩石化学组成特征为典型的钾玄质岩石，岗尾、轮水石英二长岩和花岗闪长岩属酸性—偏中性的钾玄质系列岩石（少数样品为高钾钙碱性岩石），而石花岗闪长岩则为典型的钙碱性系列岩石，这些岩体在岩石化学演化趋势上存在显著的差异，不属于同一个岩石成因系列。 40Ar- 39Ar和 Rb- Sr同位素定年结果表明，马山和岗尾岩体的形成年龄分别为 (164± 2) Ma和 (154± 1) Ma。阳春地区从中侏罗世到早白垩世期间发生了 3次主要的岩浆活动，从早到晚岩浆的 K富集程度逐渐降低，岩浆岩成分从钾玄质系列过渡到钙碱系列。

Petrological and major element geochemical features show that the Mashan Monzo- diorite, Gangwei- Lunshui Quartz Monzonite/Granodiorite and Shilu Granodiorite are of typical shoshonitic, transitional from felsic- intermediate shoshonitic to high- K calc- alkaline and typical calc- alkaline in composition, respectively. In the Hark diagram, major elements of these three series of intrusives have different evolutionary trends, indicating that they belong to different magmatic series. Hornblende 40Ar- 39Ar analysis yields a plateau age of (164± 2) Ma for the Mashan Pluton, and the Rb- Sr whole- rock and mineral isotopic data give an isochron age of (154± 1) Ma for the Gangwei Pluton. In combination with the previous radiometric dates, three major episodes of magmatism during the middle Jurassic to the early Cretaceous are identified in the Yangchun Basin. Regional Magmatic compositions changed from shoshonitic in the middle Jurassic to calc- alkaline in the early Cretaceous by decreasing K contents.