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Magmatic Differentiation and Autometasomatism in a Zoned Granitic Batholith from Central Texas, U.S.A.

1968; Elsevier BV; Linguagem: Inglês

10.1016/b978-0-08-012835-1.50070-2

Paul C. Ragland, Gale K. Billings, J.A.S. Adams,

Geology and Paleoclimatology Research

Analyses for several elements were performed on seventy-four bulk samples, thirty-two biotites, and thirty K-feldspars from the Enchanted Rock batholith, central Texas, by atomic absorption, gamma-ray spectrometry, emission spectroscopy and X-rays. The batholith is concentrically zoned from leucogranites along the margins to granodiorites toward the center. The average composition of the relatively fine-grained core falls off compositional trends for the other zones. As a result the core is considered to be a separate, later intrusive. There is no consistent gradational variation in composition within any one zone. These features, along with arcuate ribs of quartz monzonite apparently intruding the granites, may be explained by multiple injection of a granitic magma differentiating at depth rather than in situ differentiation from the interior outward. The magma is considered to be derived from material deep in the lower crust or upper mantle. The trace and minor elements analyzed may be considered on the basis of two groups: (1) the mobile elements, exemplified by Rb, U and Li, and (2) the immobile elements, Cr, Sr, Mn, Co, Ni, Ti and V. The immobile elements show the common variation of minimum concentration in the more fractionated rocks and their constituent minerals. The expected parallel decrease of K/Rb and Mg/Li ratios and increasing U concentrations is not observed. Instead the reverse is observed with the minimum K/Rb ratios and maximum U contents occurring in the granodiorites. This trend can be seen in the minerals as well as rocks and is not wholly related to any modal variation within the rocks. The Mg/Li ratios remain essentially constant throughout the batholith. These anomalous trends are explained by selective removal of the more mobile constituents by late-stage hydrothermal solutions or a vapor phase permeating the outer portions of the batholith.