ISSN:
1432-0967
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract The Trans-Pecos Magmatic Province (TPMP) is an alkalic field that was active between 48-17 Ma. Rocks of two subprovinces in the eastern alkalic belt of the TPMP, the Big Bend region and the Davis Mountains, have been analyzed for major and trace element concentrations in order to determine what magmatic processes operated to influence the compositional evolution of the magmas, and to explore what relationship existed between the silica-oversaturated evolved rocks and the silica-undersaturated mafic rocks. Similar compositional trends exist in both subprovinces, implying that the evolved rocks are genetically related to the mafic rocks, and that the differentiation processes were broadly reproducible spatially and temporally. Four stages of evolution have been identified: Stage I, alkali basalt to trachyte; stage II, trachyte to quartz trachyte; stage III, quartz trachyte to rhyolite/comendite; and stage IV, rhyolite/comendite to high-silica rhyolite/comendite. These stages were identified by discontinuities in trends on variation diagrams; within stages I and II, more than one subtrend exist. Stage I can best be modeled as the result of simple crystal fractionation with minor magma replenishment. Two subtrends within this stage indicate that variations in the processes or their rates may have occurred. Stage II exhibits both closed- and open-system behavior. The open system behavior consists of combined fractionation-assimilation and episodic mixing of stage II and stage I magmas. Stages III and IV evolved under open system processes of combined fractionation-assimilation, with the assimilant having compositional characteristics of a shale-dominated sedimentary assemblage. The four stages and subtrends within the stages occur in both subprovinces. Further, some subtrends comprise rocks that differ in age by as much as 10 m.y. In each subprovince, the stratigraphy indicates a random interlaying of rocks of the different stages, generally erupted from more than one center. That contemporaneous magmas of different stages existed in a given subprovince is indicated by the interfingering of their erupted products. These constraints argue against a single magma production-evolution scheme. Rather, the data suggest that magmas of the different stages were produced more than once during the evolution of the eastern TPMP, and that during any given time, production and evolution of magma of all stages were occurring.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00375216
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