Proterozoic thermal and deformational history and geochemical evolution of the 1.4 Ga Priest pluton and its aureole
Paleoproterozoic (1.7-1.6 Ga) supracrustal rocks in the aureole of the 1.43 +/- 0.01 ga Priest pluton provide evidence for a polyphase Proterozoic metamorphic history in central New Mexico. This history involved pre-, syn-, and post-emplacement metamorphic mineral growth. The Priest pluton contact aureole is 1.2 kilometers wide and is characterized by two isograds (first appearance of sillimanite and staurolite, respectively). Garnet-biotite temperatures combined with assemblage data (sillimanite + K-feldspar) suggest an increase from 525 degrees to ~650 degrees Celsius toward the pluton at pressures of ~4 kbar, suggesting emplacement depths of ~12 km. An 40Ar/39Ar age of 1.44+/- 0.005 Ga for hornblende in the aureole suggests rapid cooling through ~500 degrees Celsius following pluton emplacement. The well-defined thermal aureole can be used to help constrain both pre- and post-emplacement metamorphisms. Pre-pluton metamorphism is recorded by amphibolite-facies inclusions inside contact metamorphic porphyroblasts. Included minerals are interpreted to be part of a regional metamorphic assemblage because similar minerals are found outside the aureole. P-T estimates outside the aureole suggest pre-pluton regional conditions near 525 degrees Celsius and 4 kbar. Post-emplacement greenschist-facies retrograde metamorphism was superimposed on the northern and southern parts of the contact aureole and is characterized by the replacement of contact-metamorphic porphyroblasts with muscovite and/or chlorite. Retrogression was most likely due to continued cooling following pluton emplacement. 40Ar/39Ar muscovite plateau ages from the aureole range from 1.39 to 1.36 Ga. These data bracket the retrograde metamorphism between 1.43 and 1.39-1.36 Ga, and suggest that ambient temperatures following pluton emplacement were about 350 degrees Celsius for a prolonged (30 Ma) period, suggesting slow cooling following 1.4 Ga plutonism. The 1.43 Ga Priest pluton in the southern Manzano Mountains provides a key area for understanding middle crustal processes of regional aureole metamorphism and foliation reactivation and for deciphering the tectonic history of Proterozoic rocks in New Mexico. Because of its limited size (9 km^2) and well-exposed contact aureole, and because syn- and post-emplacement strain was low ("anorogenic" pluton), porphyroblast-matrix relationships can be used effectively to document pre-, syn-, and post-pluton deformations. Pre-pluton deformation is documented at the microscopic and mesoscopic scale by structures that are crosscut by the pluton or pluton-related tabular dikes and at the microscopic scale by fabrics that are overgrown by contact metamorphic porphyroblasts. Pre-pluton regional, probably recumbent, F1 folds are refolded by upright F2 folds with subvertical NE-striking axial planar cleavage (S2) which is also truncated by the pluton. Syn-pluton deformation (1.43 Ga) involved: (1) tightening of earlier F2 folds (as shown by folded dikes), (2) reactivation of S2 cleavage (as shown by syntectonic textures involving contact-metamorphic porphyroblasts), (3) formation of F3 folds in the roof of the pluton, and (4) subvertical magmatic fabric in the pluton that strikes NE, parallel to the pre-existing (S2) anisotropy in country rocks. Post-pluton emplacement deformation includes discrete mylonites in the pluton and kinks and crenulations in the aureole. Partitioned deformation and foliation reactivation during and following pluton emplacement was responsible for producing apparently ambiguous porphyroblast-matrix relationships in contact metamorphic porphyroblasts in the aureole. The northeastern margin of the pluton was a strain shadow for syn- and post-emplacement shortening and shows textures where contact-metamorphic porphyroblasts overgrow well-developed, northeast-trending, pre-pluton S2 crenulation cleavages (stage 4-5 of Bell and Rubenach, 1983). In contrast, on the northwestern side and in the roof of the pluton porphyroblasts grew during development and intensification of a northeast-striking significant syn-pluton (S2') and show typical "syn-tectonic" textures where: (1) porphyroblasts overgrow stage 2 internal crenulations whose axial plane is parallel to well-developed (stage 5) crenulation cleavage in the matrix or (2) straight to sigmoidal internal inclusion trails that are at an angle to but continuous with coarsened cleavage in the matrix indicating rotation of matrix relative to porphyroblasts during syn- and post-pluton deformation. Thus, the Priest pluton aureole offers a type example of complexities of porphyroblast-matrix relationships that develop in polymetamorphic and polydeformational terranes. Even this simple "anorogenic" mid-crustal pluton emplacement was accompanied by partitioned mid-crustal flow that records the combined effects of thermal softening and partitioned regional deformation. The Priest pluton is part of the suite of 1.4 Ga granitic plutons in North America. The pluton is a corundum normative, peraluminous, K-feldspar megacrystic granodiorite to quartz monzonite. The Priest pluton is characterized by high Mg/Mg+Fe and high abundances of Al2O3, CaO, and large ion lithophile elements (e.g. Zr, Y). Major and trace element modeling suggests that variations in composition were produced by the accumulation of K-feldspar and plagioclase, accumulation of plagioclase, biotite, and accessory minerals, and crystal fractionation of the residual melt. Plutons in the suite of 1.4 Ga, so-called "anorogenic", granitic plutons have been classified as A-type granites. Major and trace element abundances suggest that the Priest pluton does that have an A-type granite signature but has more in common with S-type granites. In addition, typical tectonic classification techniques suggest that the pluton is either a volcanic-arc/syn-collisional or late orogenic granite, not an "anorogenic" granite. The Priest pluton is also different from the nearby Sandia pluton, although the two plutons are nearly the same age. The differences in geochemistry between the Priest and Sandia plutons and the rest of the suite of 1.4 Ga granitic plutons suggest that the lower crust is a heterogeneous source region. Therefore, geochemical signatures that are commonly associated with particular tectonic settings may instead be a reflection of heterogeneities in the source.
Proterozoic, Proterozoic Deformational History, Proterozoic Thermal History, Proterozoic History, Geochemical Evolution, Priest Pluton
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