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P‐T‐X conditions of calc‐silicate formation: evidence from fluid inclusions and phase equilibria; Llano Uplift, central Texas, USA
Author(s) -
LETARGO C. M. R.,
LAMB W. M.
Publication year - 1993
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1993.tb00133.x
Subject(s) - geology , grossular , geochemistry , metamorphic rock , fluid inclusions , silicate , quartz , gneiss , metamorphic facies , calcite , pluton , mineralogy , facies , geomorphology , chemistry , paleontology , organic chemistry , structural basin , tectonics
The Llano Uplift in central Texas is a Grenville aged ( c. 1.1 Ga) metamorphic terrane consisting predominantly of amphibolite facies mineral assemblages. The formation of these assemblages has been attributed to the emplacement of relatively late granite plutons throughout the area. Two types of granitic intrusion have previously been recognized: (1) Town Mountain Granites, which occur as relatively large, circular‐shaped bodies of coarse‐grained granite, and (2) Younger Granites which are present as smaller and more irregular bodies of finer‐grained granite. In the central part of the uplift, wollastonite‐bearing calc‐silicate rocks occur within the Valley Spring Gneiss. The development of these calc‐silicate rocks has been linked to infiltrating fluids presumably derived from spatially associated Younger Granites. The stability of coexisting quartz, calcite, wollastonite, grossular and anorthite and coexisting quartz, calcite, wollastonite, andradite and hedenbergite shows that the calc‐silicate rocks equilibrated under H 2 O‐rich conditions with χCO 2 <0.10. Fluid inclusions present within the calc‐silicate minerals are H 2 O‐rich with salinities of <17 wt% equivalent NaCl. The absence of any detectable CO 2 in the fluid inclusions may indicate entrapment of the inclusions at lower pressures and more H 2 O‐rich conditions compared to the stability of the peak metamorphic mineral assemblage. Homogenization temperatures, measured for texturally primary inclusions, range from 360 to 368° C corresponding to a density range from 0.53 to 0.82 g/cm 3 . Isochores for these fluid inclusions, when combined with the stability of the solid‐solid equilibria Grs + Qtz = Wo + An, yield formation conditions of 500–550° C at 1–2 kbar. This indicates that the granitic intrusions involved in the formation of the Blount Mountain calc‐silicates were emplaced at a pressure of at least 1–2 kbar.