The central feature of the range, as worked out by Mr. Tolman, is a great post-Carboniferous intrusive mass of siliceous muscovite granite modified to a gneissic rock near its margins, surrounded by an intense contact metamorphism in which rocks of widely different kinds have been conspicuously affected. The oldest rock cut by this granite is a coarse biotite granite which apparently as a result of the later granitic intrusion, grades into augen gneiss, and locally this rock in turn has been transformed into a thinly fissile schist. ABSTRACTRecent field work and accumulated Rb-Sr studies, when combined with previous U-Th-Pb and K-Ar investigations, allow a new synthesis of the crystalline terrane within the Santa Catalina-Rincon-Tortolita crystalline complex. When all the available data are integrated, it is apparent that the crystalline core is mainly a composite batholith that has been deformed by variable amounts of cataclasis. The batholith was formed by three episodes of geologically, mineralogically, geochemically, and geochronologically distinct plutons. The first episode (75 to 60 m.y. B.P.) consisted of at least two 217 on June 19, 2015 memoirs.gsapubs.org Downloaded from of these events is clearly constrained to the Catalina intrusive episode because it formed during or after the emplacement of Tortolita quartz monzonite (about 27 m.y. B.P.) but before the intrusion of postfoliation dikes (about 24 m.y. B.P.). All three episodes of mylonitization contain the distinctive and much discussed east-northeast-trending lineations. All events of mylonitization are constrained to a 50-m.y. interval of time from 70 to 20 m.y. ago. Although continuous mylonitization from 70 to 20 m.y. ago cannot be unequivocally disproved, the strong association of mylonitization with the three plutonic episodes suggests that deformation in the Santa Catalina-Rincon-Tortolita crystalline core, like intrusion, was episodic. on June 19, 2015 memoirs.gsapubs.org Downloaded from NviyaiAivoaud A / \ t S f J tu V oo > e 1
K and Ar analyses on cogenetic feldspars and micas from four plutonic bodies, two pegmatites, and one metamorphic terrain indicate that (1) albite and plagioclase have equal or greater argon retentivity than their cogenetic micas, (2) some potash feldspars of Tertiary age may quantitatively retain argon, and (3) intense metamorphic processes can thoroughly degas both feldspars and micas. The K‐Ar ages of the micas and known chronostratigraphic relations indicate that pegmatitic albite, Tertiary plutonic plagioclase, and possibly Tertiary plutonic potash feldspar contain Ar40 in excess of that generated by the decay of their contained K. This excess component is believed to have been occluded within these minerals at the time of their crystallization from the liquid magma. The excess Ar40 contents derived from these measurements is 0.15 to 2.71 × 10−10 mole/gram in albites and plagioclases of Paleozoic and Tertiary age. The amount of excess Ar40 most likely reflects the partial pressure of Ar40 in the lithospheric gases. K‐Ar ages of plagioclase from two Precambrian quartz monzonites are very nearly the same as the K‐Ar ages of cogenetic biotite and muscovite. These results are concordant with Rb‐Sr whole‐rock isochron and mica ages of 1420 m.y. and demonstrate that plagioclase from Precambrian acidic plutonic rocks can be as precise a chronometer as mica minerals.
Stratified sequences of Precambrian rocks from twelve localities in Arizona and Sonora yield geologic and isotopic evidence of multiple periods of rock accumulation and orogeny. Isotopic dating indicates that these rocks were formed from 1800 m.y. to 950 m.y. ago. The oldest rocks are metamorphosed sequences of volcanic and sedimentary origin. They are separated in time from a younger sequence of metamorphosed volcanic and sedimentary rocks by batholithic plutonism dated at approximately 1700 m.y. The second period of volcanism and sedimentation was terminated by batholithic plutonism about 1400 m.y. ago.Subsequent to the 1400 m.y. event, the "Younger Precambrian" rocks of Arizona, the Apache Group and probably the Grand Canyon Series, were deposited. These dominantly sedimentary rocks are characteristically only slightly and locally metamorphosed and deformed. Their age is greater than the 1150 m.y. old diabase sills and dikes that intrude them. Some areas that are not underlain by rocks of the Apache Group were metamorphosed and possibly intruded by silicic plutons about 1000 m.y. ago.From these data it appears that the stratified Precambrian rocks of Arizona can be assigned to three periods of accumulation. The first period occurred prior to about 1700 m.y., and the second occurred between 1700 and 1400 m.y. The third period, the deposition of the Apache Group, occurred between 1400 and 1150 m.y. ago.
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