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Title: Use of porphyroblast-matrix relationships to solve a complex polyphase geologic history in Middle Proterozoic rocks of central New Mexico

Abstract

Porphyroblast-matrix textural relationships are often ambiguous and, used in isolation, can lead to conflicting interpretations of the geologic history of an area because deformation and metamorphism are generally heterogeneous in both space and time. This is true in Proterozoic rocks in southwestern North America where the debate about an orogenic versus an orogenic Middle Proterozoic geologic history continues. For example, the emplacement of the 1.43 Ga Priest quartz monzonite in central New Mexico is here interpreted to have post-dated most of the region (D[sub 2]) deformation and some amphibolite-facies metamorphism, to have been synchronous with continued or renewed NW-SE shortening and development of a 1--2 km wide metamorphic aureole, and to have been overprinted by low-temperature deformation after cooling. Apparently conflicting porphyroblast-matrix assemblages and disequilibrium mineral assemblages can be reconciled with this polyphase history. Evidence that a major component of the NW-SE shortening pre-dated pluton emplacement is; (1) the pluton cross-cuts the regional, subvertical S[sub 2] fabric and isoclinal F[sub 2] folds; (2) the pluton is generally unfoliated compared to intensely deformed country rocks; (3) most contact-metamorphic minerals (75%) epitaxially overgrow a differentiated S[sub 2] cleavage. Pre-pluton metamorphism may be represented by kyanite, rare staurolites that are included in contact-metamorphicmore » garnets, and cloudy, anhedral garnets that are locally overgrown by contact-metamorphic garnets.« less

Authors:
;  [1]
  1. (Univ. of New Mexico, Albuquerque, NM (United States). Dept. of Earth and Planetary Sciences)
Publication Date:
OSTI Identifier:
5130052
Report Number(s):
CONF-9305259--
Journal ID: ISSN 0016-7592; CODEN: GAAPBC
Resource Type:
Conference
Resource Relation:
Journal Name: Geological Society of America, Abstracts with Programs; (United States); Journal Volume: 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993
Country of Publication:
United States
Language:
English
Subject:
58 GEOSCIENCES; GEOLOGIC FORMATIONS; PARAGENESIS; PETROGENESIS; NEW MEXICO; PLUTONIC ROCKS; DEFORMATION; GEOLOGIC HISTORY; METAMORPHISM; MINERALOGY; PETROLOGY; PRECAMBRIAN ERA; DEVELOPED COUNTRIES; GEOLOGIC AGES; GEOLOGY; IGNEOUS ROCKS; NORTH AMERICA; ROCKS; USA 580000* -- Geosciences

Citation Formats

Thompson, A.G., and Karlstrom, K.E.. Use of porphyroblast-matrix relationships to solve a complex polyphase geologic history in Middle Proterozoic rocks of central New Mexico. United States: N. p., 1993. Web.
Thompson, A.G., & Karlstrom, K.E.. Use of porphyroblast-matrix relationships to solve a complex polyphase geologic history in Middle Proterozoic rocks of central New Mexico. United States.
Thompson, A.G., and Karlstrom, K.E.. 1993. "Use of porphyroblast-matrix relationships to solve a complex polyphase geologic history in Middle Proterozoic rocks of central New Mexico". United States. doi:.
@article{osti_5130052,
title = {Use of porphyroblast-matrix relationships to solve a complex polyphase geologic history in Middle Proterozoic rocks of central New Mexico},
author = {Thompson, A.G. and Karlstrom, K.E.},
abstractNote = {Porphyroblast-matrix textural relationships are often ambiguous and, used in isolation, can lead to conflicting interpretations of the geologic history of an area because deformation and metamorphism are generally heterogeneous in both space and time. This is true in Proterozoic rocks in southwestern North America where the debate about an orogenic versus an orogenic Middle Proterozoic geologic history continues. For example, the emplacement of the 1.43 Ga Priest quartz monzonite in central New Mexico is here interpreted to have post-dated most of the region (D[sub 2]) deformation and some amphibolite-facies metamorphism, to have been synchronous with continued or renewed NW-SE shortening and development of a 1--2 km wide metamorphic aureole, and to have been overprinted by low-temperature deformation after cooling. Apparently conflicting porphyroblast-matrix assemblages and disequilibrium mineral assemblages can be reconciled with this polyphase history. Evidence that a major component of the NW-SE shortening pre-dated pluton emplacement is; (1) the pluton cross-cuts the regional, subvertical S[sub 2] fabric and isoclinal F[sub 2] folds; (2) the pluton is generally unfoliated compared to intensely deformed country rocks; (3) most contact-metamorphic minerals (75%) epitaxially overgrow a differentiated S[sub 2] cleavage. Pre-pluton metamorphism may be represented by kyanite, rare staurolites that are included in contact-metamorphic garnets, and cloudy, anhedral garnets that are locally overgrown by contact-metamorphic garnets.},
doi = {},
journal = {Geological Society of America, Abstracts with Programs; (United States)},
number = ,
volume = 25:5,
place = {United States},
year = 1993,
month = 4
}

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  • Distinctive lithostratigraphic markers, metamorphic isobaric surfaces, major ductile thrusts and overturned folds in Early Proterozoic rocks from 4 isolated uplifts in north-central NM provide relatively firm piercing points for restoration of over 50 km of right lateral strike-slip movement along a network of N-S trending faults. In addition, the authors speculate that the Uncompahgre Group in the Needle Mts. of southern Colorado is correlative with the Hondo Group in northern NM; suggesting over 150 km of right-lateral strike slip offset has occurred across a network of N-S trending faults that includes the Picuris-Pecos fault, the Borrego fault, the Nacimiento faultmore » and others. The tectonic implications of this reconstruction span geologic time from the Proterozoic to the Cenozoic. The restoration of slip provides new insights into the structure of the Proterozoic basement in NM. Volcanogenic basement (1.74--1.72 Ga) and overlying sedimentary cover (Hondo Group) are imbricated in an originally EW- to NW-trending ductile foreland thrust and fold belt that formed near the southern margin of 1.74--1.72 basement. The authors propose that the volcanogenic basement rocks correlate with rocks of the Yavapi Province in Arizona and that the Hondo Group correlates with foreland rocks of the Tonto Basin Supergroup. Rocks south of this belt are 1.65 Ga or younger and are interpreted to belong to a separate crustal province which correlates with the Mazatzal Province in Arizona. Proterozoic ductile fault geometries suggest that the Mazatzal Province was thrust northward and resulted in imbrication of Yavapi Province basement and its siliciclastic over sequence.« less
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  • Ductile thrust faults truncate contact-metamorphic aureoles surrounding two 1.4 Ga plutons in central New Mexico. The Priest quartz monzonite (1440 Ma) and Sandia granite (1420 Ma) are 50 km apart in the continuous Sandia/Manzano mountain chain. Thermobarometry and phase relations demonstrate that country-rock temperatures rose from 700 C toward the pluton, at pressure near 4 kb. The northern edge of this aureole is cut by the southeast-dipping ductile Monte Largo thrust fault. Prograde, greenschist-facies metamorphism of footwall rocks accompanied local retrogression of hangingwall rocks during thrusting. This second metamorphism took place at P-T conditions of 2-3 kb and 450-475 C.more » Another contact aureole surrounds the Sandia granite. Mafic rocks near the granite reflect amphibolite-facies conditions, whereas pelites display low-pressure contact assemblages. Quantitative temperatures increase from 500--750 C toward the granite, at pressures of 2.5--3.5 kb. The shallowly southeast-dipping Vincent Moore fault truncates the Sandia granite and the southern portion of its contact aureole. This ductile shear zone emplaced greenschist-facies rocks northwestward above the Sandia contact aureole. Footwall rocks were retrograded to the greenschist facies within 100 m of this fault; the retrograde phases are aligned parallel to the trace of the thrust. Metamorphic temperatures in hangingwall rocks (during thrusting ) were 400-475 C at pressures above 2.75 kb. Additional northwest-vergent ductile thrusts are found elsewhere in the mountain chain. This may represent the age of thrusting and of the related greenschist and the related greenschist-facies metamorphic overprint.« less
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