U-Pb Geochronology of Monazite and Zircon in Precambrian Metamorphic Rocks from the Ruby Range, SW Montana PDF Download

Author: Carson L. Jones
Publisher:
ISBN:
Category : Geological time
Languages : en
Pages : 119

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Book Description
U-Pb accessory minerals in metamorphic rocks from the Ruby Range, southwestern Montana, were analyzed isotopically and chemically by microprobe techniques in order to constrain the timing of Precambrian thermotectonic events that affected the NW edge of the Wyoming craton. Dated samples represent the Cherry Creek Metasupracrustal Suite and associated pegmatites (CCMS, youngest), the Dillon quartzofeldspathic gneiss (DQG), and the Pre-Cherry Creek basement gneisses (PCCG, oldest). Undeformed bodies of pegmatite cross-cut the NE-trending fabric of the CCMS, and coexisting monazite and xenotime from the pegmatite yield crystallization ages of 1762 " 6 Ma and 1754 " 6 Ma, respectively (207Pb/206Pb upper-intercept ages reported at 95% confidence). These results constrain latest magmatism to within a ~1760 Ma time frame, while also establishing this as a minimum age for the cross-cut fabric. Metamorphic monazite in CCMS metapelites formed at 1814 " 15 and 1773 " 8 Ma, whereas monazites preserved as inclusions in CCMS garnet record a much older ~2450 Ma event (total-Pb ages reported at 95% confidence). In DQG samples that also contain ~2450 Ma monazite in garnet, matrix monazite preserves mixed ~1760-1580 Ma dates. In one DQG sample, polymetamorphic matrix monazite formed at 1784 " 57 and 2468 " 11 Ma, whereas older magmatic zircon formed at 2772 " 11 Ma. Polygenetic zircon in another DQG sample formed at both 2471 " 20 Ma (metamorphic; Th/U = 0.0-0.1) and at 2772 " 7 Ma (magmatic; Th/U = 0.5-0.7). Likewise, zircon in two PCCG samples preserves mostly 2437 " 47 and 2762 " 37 Ma ages in rims and cores, respectively, but also exhibits subordinate, inherited cores of ~3000-3500 Ma age. Indications of a ~2550-2500 Ma event in the Ruby Range are represented by a single total-Pb date of monazite (inclusion in DQG garnet) and by a population of PCCG metamorphic zircon dated at 2523 " 14 Ma (upper-intercept age). The mixed ~1760-1580 Ma dates observed for monazite in one DQG migmatite are of uncertain origin. Possible hypotheses include rifting related to the development of the Belt Basin or to younger break-up and dispersal of supercontinent Rodinia. The ~1760 Ma mineral ages date the waning phases of thermotectonism associated with the Wyoming-Medicine collision along the Great Falls tectonic zone, which spanned an ~1865-1760 Ma interval. Monazite growth ages of ~1815-1770 Ma document earlier stages of this collision. The ~2460 Ma mineral ages reflect an earlier, cryptic thermotectonic event that is temporally associated with incipient breakup of the ~2700-2500 Ma supercontinent Kenorland. The ~2550 Ma ages may record the juxtaposition of the Montana metasedimentary terrane as part of the Wyoming craton, along the NE-trending Madison mylonite zone, during terminal assembly of Kenorland. Finally, occurrences of ~2780 Ma mineral ages in the DQG record the Beartooth orogenic signature, whereas preservation of ~3000-3500 Ma zircon ages documents a Pre-Beartooth history for the PCCG basement.