The Journal of Petrology, Volume 38, Issue 10: October 1997.
Grenville magmatism in West Texas: petrology and geochemistry of the Red Bluff granitic suite
WM. Shannon1,3, CG. Barens1,* and ME. Bickford2
1Department of Geosciences, Texas Tech University, Lubbock, TX 79409-1053, USA and
2Department of Earth Sciences, Syracuse University, Syracuse, NY 13244-1070, USA
3Present address: 9180 Coors Blvd., NW, Apt. 3606, Albuquerque, NM 87120, USA
*Corresponding author e-mail: gical@ttacs.ttu.edu
ABSTRACT
The Red Bluff granitic suite (RBGS) underlies part of the Franklin Mountains in west Texas. It was emplaced into a Middle Proterozoic shelf sequence at 1120+-35 Ma. The suite is predominantly metaluminous; it consists of early granitic to quartz syenitic sills, a main mass of alkali feldspar granite, small bodies of alkali feldspar syenite, leucogranitic dikes, and late-stage peralkaline arfvedsonite granite. Late-stage, mildly alkaline (transitional) ferrobasaltic dikes are also present. The suite shows continuous compositional variation from quartz syenite to leucogranite. It is characterized by an iron-rich mineral assemblage, estimated f(O2) near FMQ, initial emplacement temperatures near 1050oC, and high concentrations of high field strength and rare earth elements. These features serve to classify the suite as A-type; trace element discrimination diagrams identify it as 'within plate'. When plotted in multielement diagrams, trace element patterns of both the granitic rocks and the ferrobasalts are generally subparallel to those of ocean island basalts. No elemental evidence for a subduction-related origin is present among the granitic rocks. Major element mass-balance models that use observed phases are consistent with an origin by fractional crystallization from a transitional ferrobasaltic parent. However, trace element tests of these models indicate that trapped liquid remained in the crystalline residue ('in situ crystallization'). Partial melting models that use known basement rocks as parental compositions generally fail to reproduce the observed trace element patterns, and those that fit individual rock compositions cannot explain the continuous compositional variation of the suite. Derivation from basaltic parental magmas indicates theat the RBGS represents addition of juvenile material to the crust during Grenville time. The mildly alkaline nature of the granitic suite and its lack of subduction-related geochemical features are consistent with an origin in a zone of regional extension, as suggested by recent geophysical and petrological studies.
Keywords: alkaline granite; geochemistry; Grenville; geochronology; Texas
Pages 1279-1305
This page is run by
Oxford University Press,
Great Clarendon Street, Oxford OX2 6DP,
as part of the
OUP Journals
World Wide Web service.
Comments and feedback:
www-admin@oup.co.uk
URL: http//www.oup.co.uk/petroj/hdb/Volume_38/Issue_10/381279.sgm.abs.html
Last modification: 30 October 1997.
Copyright©
Oxford University Press, 1997.