Petrogenesis of Triassic Intrusions, SW Maine: Forerunners of WMMS Igneous Activity

新英格兰中生代非造山火成岩活动是一次长寿、岩石学变化多样的岩浆活动。岩浆作用的三个主要脉冲被识别为：(1)早三叠世阶段，(2)空间受限的大量侏罗纪脉冲，(3)区域广泛的白垩纪阶段。现代岩石学研究集中在后两个阶段；岩石成因结论从简单的地壳深熔作用到镁铁质地幔岩浆的地壳同化和分离结晶(AFC)的组合。相比之下，最早的活动阶段，即三叠纪事件，在岩石化学上被忽视了。仅在缅因州西南部暴露，这一活动主要由Mt.阿伽门托斯杂岩(碱性正长岩到花岗岩)和阿伯特山股票(正长岩)。利用现代岩石化学技术和地质年代学(相岩石学、主量元素、痕量和稀土元素地球化学、锶、铅、钕同位素地球化学和40Ar/39Ar同位素年代学)，该研究将仔细描述这些侵入岩的特征，并开发全面的岩石成因解决方案。初步结果表明，镁铁质岩浆的地壳深熔作用而不是AFC作用在这些深成岩类的成因中起着重要作用。如果是这样的话，那么这一三叠纪岩浆事件可能类似于晚侏罗世的脉冲，可以被认为是该活动的先驱。对所有中生代火成事件的岩石成因模型进行比较，可以加深我们对这种“非造山”岩浆活动演化的理解，并对作为最终原因的构造/热活动的性质提供更好的约束。

Anorogeneic igneous activity during the Mesozoic in New England was a long-lived and petrologically varied magmatic event. Three major pulses of magmatism are recognized: (1) an early Triassic phase, (2) a spatially restricted and voluminous Jurassic pulse, and (3) a regionally extensive Cretaceous phase. Modern petrologic investigations have concentrated on two later phases; petrogenetic conclusions vary from simple crustal anatexis to combined crustal assimilation and fractional crystallization (AFC) of mafic, mantle-derived magmas. In contrast, the earliest phase of activity, the Triassic event, has been petrochemically ignored. Exposed only in SW Maine, this activity is primarily represented by the Mt. Agamenticus complex (alkaline syenite to granite) and the Abbott Mountain stock (syenite). Using modern petrochemical techniques and geochronology (phase petrology, major, trace, and rare earth element geochemistry, Sr, Pb, and Nd isotope geochemistry, and 40Ar/39Ar isotope geochronology) the proposed research will carefully characterize these intrusions and develop comprehensive petrogenetic solutions. Preliminary results suggest that crustal anatexis and not AFC of mafic magmas is important in the genesis of these plutons. If so, then this Triassic magmatic event may be similar to the later Jurassic pulse and can be considered the forerunner of that activity. Comparison of petrogenetic models developed for all of the Mesozoic igneous events should increase our understanding of the evolution of this 'anorogenic' magmatism and provide better constraints on the nature of the tectono/thermal activity that was is ultimate cause.