Volume 38: January - December 1997

Issue 3: March 1997

Abstract

Enclaves in the Cadillac Mountain granite (coastal Maine): samples of hybrid magma from the base of the chamber

RA. Wiebe, D. Smith², M. Sturm³, EM. King⁴ and MS. Seckler⁵ ¹Department of Geosciences, Franklin and Marshall College, Lancaster, PA 17604, USA, ²Department of Geosciences, Trinity University, San Antonio, TX 78212, USA, ³Department of Geology, Duke University, Durham, NC 27706, USA, ⁴Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706, USA, ⁵Department of Geology, Williams College, Williamstown, MA 01267, USA and Corresponding author. Present address: School of Earth Sciences, Macquarie University, N.S.W. 2109, Australia
ABSTRACT
The Cadillac Mountain intrusive complex is dominated by the Cadillac Mountain granite and a 2-3 km thick section of interlayered gabbroic, dioritic and granitic rocks which occurs near the base of the granite. The layered rocks record hundreds of injections of basaltic magma that ponded on the chamber floor and variably interacted with the overlying silicic magma. Magmatic enclaves, ranging in composition from 55 to 78 wt % SiO2, are abundant in granite above the layered mafic rocks. The most mafic enclaves are highly enriched in incompatible elements and depleted in compatible elements. Their compositions can be best explained by periodic replenishment, mixing and fractional crystallization of basaltic magma at the base of the chamber. The intermediate to silicic enclaves formed by hybridization between the evolved basaltic magma and resident silicic magma. There is little evidence for significant exchange between enclaves and the enclosing granite. Instead, hybridization apparently occurred between stratified mafic and silicic magmas at the base of the chamber. Enclaves in a restricted area commonly show distinctive compositional characteristics, suggesting they were derived from a discrete batch of hybrid magma. Enclaves were probably dispersed into a localized portion of the granitic magma when replenishment or eruption disrupted the intermediate layer.

Keywords: granite; enclaves; mafic-silicic layered intrusion; fractional crystallization; zoned magma chamber

Pages: 393 - 423

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