Model for the Origin and Significance of Microgranular Enclaves in Calc-alkaline Granitoids |
| |
| Authors: | POLI GIAMPIERO E; TOMMASINI SIMONE |
| |
| Institution: | Dipartimento Scienze della Terra, Universit? di Perugia, P. za Universit? 06100—Perugia, Italy |
| |
| Abstract: | The origin and significance of microgranular enclaves as indicatorsof the occurrence of magma mixing and/or mingling episodes betweenbasic magmas and anatectic acid magmas, either I- or S-type,are re-evaluated. A model for studying microgranular enclaveand host rock associations has been developed, based on thegeochemical characteristics and the outcrop relationships observedin microgranular enclave-bearing granitoid suites. The model consists of three main stages, concerning the injection(stage 1), the evolution (stage 2), and the mixing (stage 3)processes that basic magmas experience when injected into anatecticcrustal environments. In stage 1, an acid magma is intrudedby one or more injections of almost completely liquid basicmagma which is hotter and less viscous than the acid magma.The two systems do not mix easily, but remain as discrete entitiesuntil thermal equilibrium and comparable viscosities are reached,and freezing of the basic magma and superheating of the acidmagma are operative along their boundaries. In stage 2, thebasic magma experiences both the physical processes of stretching,convective stirring, and mingling with the acid magma, and thechemical processes of crystal fractionation, and contaminationwith the acid magma (CFC process). Repeated cycles of thesephysical and chemical processes result in the formation of bothmicrogranular enclaves and an evolved liquid which is thermallyequilibrated with the acid magma. Accordingly, microgranularenclaves record the steps of evolution of the basic magma. Instage 3, the evolved products of the basic magma (tonaliticto granodioritic in composition) and the acid magma participatein a two-endmember mixing process which accounts for the geochemicalevolution of granitoid plutons bearing microgranular enclaves. The model sheds new light on the magmatic processes occurringin plutonic environments during the formation of composite batholiths,and also suggests some ideas on the petrogenesis of tonaliticplutons. Finally, the observed scale-independent property ofmicrogranular enclaves suggests that fractal geometry, a relativelynew topic of mathematics, can play a determinant role in theunderstanding of the chaotic flow mechanics of viscous fluids,i.e., the kinematics of the mingling and stretching of basicmagmas. |
| |
| Keywords: | |
| 本文献已被 Oxford 等数据库收录！ |
|
