Abstract:
Data are reported on more than 100 samples of rock-forming quartz from igneous rocks of associations of different ages in the Greater Caucasus and the Sierra Nevada batolith, USA. These rocks were emplaced in different geodynamic settings. Analysis of accessory minerals and the geochemistry of the rocks shows that the Ge content and the amount of index accessory Ge-bearing minerals gradually increase from the older to the younger rocks and in the final phases of the differentiated associations. Our EPR spectroscopic results show that the content of structural Al and Ge admixtures in the rock-forming quartz increases from the older to the younger associations. The lowest contents of the structural admixtures are detected in quartz of the oldest associations, which experienced multiple regional metamorphic transformations under amphibolite- and greenschist-facies conditions. Our study of the differentiated igneous complexes shows that the content of Al and Ge admixtures in the rock-forming quartz increases from the earlier to later phases of the rocks. These variations are related to the evolution of silicic magma composition, e.g., to the increasing content of lithophile elements, particularly lithium, which is the main charge-compensating element for the heterovalent substitution of Al3+ for Si4+.