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GEOLOGY OF PAPOOSE FLAT PLUTON, INYO MOUNTAINS, CALIFORNIA
Arthur Gibbs Sylvester
Last modified 1 December 2008
Thousands of scientific papers have been written about granite; they center around three main questions:
Not one of the thousands of authors of those scientific papers knows the answer to any or all of the questions for sure, because not one of them was there when the plutons intruded. Those authors can only make inference from equivocal field data, from time-limited laboratory experiments, and from questionable modeling studies. I view granite studies akin to those of a pathologist: From the corpse, s/he must infer its death, but from the granite, the geologist must infer its entire life history – its birth, its adolescence, and its death history.
Given that rather negative introduction, however, considerable insight really has been gained from the totality of field, laboratory, and modeling studies over the last 300 years. The study of granite has progressed greatly even since H. H. Read's 1947 pronouncement that "there are granites and granites".
This web site presents illustrations from studies of the now classic Papoose Flat pluton, which offers a rather clear, but not entirely unequivocal, picture of its emplacement mechanism.
ALL TOO BRIEF OVERVIEW OF EMPLACEMENT MECHANISMS
Granite bodies are very large, on the scale of tens to hundreds of cubic kilometers, large enough to require some unique process to replace the pre-existing rocks. The problem is one of transfer of mass – of a great volume of granite magma from depth into the shallow crust. The main ideas that authors over the years have proposed to explain how granite bodies make room for themselves are:
My colleagues and I concluded that Papoose Flat pluton represents a single batch of magma that intruded as a large dike, through old rocks now exposed at the east end of the pluton, and then it expanded and forcibly shoved aside younger rocks by plastic flow and recrystallization at the west end of the pluton, much like air expands a balloon. Thus, just as the balloon's walls stretch and thin, so also did the rocks around the west end of Papoose Flat pluton.
PAPOOSE FLAT PLUTON PARTICULARS – BEWARE! MUCH ESOTERIC JARGON FOLLOWS
The pluton lies at the crest of the northern Inyo Mountains between Owens and Saline valleys nearly at the latitude of Tinemaha Reservoir. Its westernmost exposure at 1830 m asl almost reaches the upper slopes of Owens Valley. The highest part of the pluton underlies Waucoba Mountain at 3390 m asl. The pluton is well exposed so that the geologic relationships leading to the conclusions about forcible emplacement, stretching and thinning of wall rocks, are readily seen in accessible outcrops.
Papoose Flat itself is underlain by granite
at an elevation of 2650 m. Several erosional remnants of granite make
scenic, randomly isolated spires, or monadnocks, on the flat that are
the object of curiosity for off-road excursionists and geologists alike.
Papoose Flat pluton is one of several granitic plutons in the White-Inyo Range of eastern California that are generally regarded as satellites of the great Sierra Nevada batholith, which lies 20 km to the west. The granitic compositions and Jurassic-Cretaceous ages of the White-Inyo plutons are closely similar to those in the Sierra. The fundamental difference is that the White-Inyo plutons are largely encased in sedimentary country rocks whose ages and former stratigraphic geometry are known or confidently inferred; only a few screens and pendants of former country rocks are found in the Sierra. There plutons intruded each other in most instances, so it is hard to know how any given pluton made room for itself. Thus it is possible to infer the damage the White-Inyo plutons did to the country rocks more confidently than for the Sierran plutons.
The country rocks in the White-Inyo Range, although extensively folded and faulted, are not metamorphosed except around plutons. Thus, the metamorphism and associated structures date from the intrusion of the plutons. This is a fundamentally important conclusion, because in so many other plutonic terranes, plutons intruded rocks that had already experienced multiple metamorphic events, so that only with great difficulty may one unravel the metamorphism and deformation associated solely with pluton emplacement. Not so at Papoose.
The unique geologic features of Papoose Flat pluton may be tabulated as follows:
Differences between East and West Halves of Papoose Flat Pluton
WHY IS PAPOOSE FLAT PLUTON UNIQUE?
feature of Papoose Flat pluton is the attenuation
of country rocks around the west half of the pluton to about ten percent
of their regional stratigraphic thickness. For example, the Poleta
Formation has a regional stratigraphic thickness of 365 m, whereas it
is less than 61 m thick around the west end of the pluton.
The metamorphic rocks around the west half of the pluton are strongly
foliated with a crystal lineation that lies within the plane of the foliation.
Rocks having pronounced rheologic differences, such as quartzite and quartz-mica
schist, are boudinaged or exhibit pinch and
swell structures in response to the relative brittle behavior of the
quartzite and the ductile behavior of the schist. The geometric relationship
of the lineation and foliation to the boudins and pinch and swell structures
clearly demonstrates that the lineation in both the outer marginal facies
of the granite and in the country rocks is a stretching lineation (an
A lineation in the strain terminology).
TEMPERATURE AND PRESSURE OF EMPLACMENT
Matt Nyman and colleagues performed detailed thermometry on the rocks and came up with thermal profiles (plots of metamorphic temperature versus distance) across the aureole that have relatively flat and narrow temperature gradients (<100m). The gradients near the contact indicate temperature decreased slightly from 500–550°C at the pluton/wall rock contact to 450–500°C at the outer margin of the aureole. The thermal effects from emplacement extend no farther than 600 m from the contact in one of the thermal profiles. The pressure was less than 4 kbar, based on the coexistence of andalusite and cordierite in pelitic rocks of the aureole. That pressure is consistent with our inference of 2-3 kbar based on the postulated stratigraphic thicknesses of rocks that probably existed atop the pluton when it was emplaced.
Nyman et al used an analytical solution of the conductive heat flow equation for a rectangular-shaped pluton to reproduce the observed thermal maxima and profile shape. They concluded that isotopic and field evidence indicates limited fluid flow along the strongly deformed margin of the pluton to conductive rather than convective cooling. They also concluded that the strain rate in the aureole rocks may have been 10-12s-l, based on simple thermal models, observed high-temperature deformation features, and a measured 90% attenuation of stratigraphic units in the plastically deformed western part of the pluton's aureole. They postulated that two distinct generations of andalusite growth in pelites is evidence for episodic heating or prolonged heating and, therefore, slower strain rates. Thermal models also indicate that parts of the pluton still may have been above the solidus during deformation of the pluton margin and aureole.
The pluton contact is cut by a set of conjugate faults, wherein the more common is the NE-striking, left-slip one. Palinspastic restoration of the faults yields a pluton shape that trends northwest in contrast to its present E-W orientation. The faults die out in the pluton where they are healed by K-feldspar and quartz, indicating that the faults formed when the margin of the pluton was cool enough to fracture, but while the core was still hot enough to yield hydrothermal, mineral-rich fluids that could heal the fractures.
We concluded that Papoose Flat pluton intruded the core of the Inyo anticline as a dike, now exposed as the discordant apophysis at the east end of the pluton. The magma rose higher into the anticline, expanded its wall rocks concordantly like air expands the skin of a balloon, and then it was cut by a set of post-crystalline, conjugate faults.
We explained the disparate behavior of country rocks in the east and west halves of the pluton as due to hydrolytic weakening of silicate minerals in arenaceous and argillaceous sedimentary rocks that, when metamorphosed, allowed them to be as rheologically ductile as calcareous rocks at the same temperatures and pressure. We noted that the country rocks around the east half of the pluton consist of notably anhydrous argillite, dolomite, and quartzite of the Wyman, Reed, Deep Spring, and lower Campito formations, whereas the upper Campito, Poleta, and Harkless formations around the west half are shale-rich. Metamorphic dehydration of the shale would yield H2O that under elevated pressure and temperature would enter the crystal lattices of silicate minerals and thereby weaken the shale-bearing formations so that they could readily flow.
Some years later, Rick Law and Sven Morgan did a detailed study of quartz and andalusite crystal fabrics in the metamorphic aureole and concluded that initial the magma intruded initially as an inclined sill that subsequently inflated into a pluton or laccolith. Their conclusion differs in being able to put a label on the resultant pluton shape by saying it is a sill or laccolith rather than some kind of abstract balloon.
Law and Morgan then teamed up with Michel de St. Blanquat and Jean-Luc Bouchez in an AMS study of the pluton's cryptic internal magnetic fabric and found that conforms to the contact and structure of the wall-rocks in the west half, supporting the pluton's concordant emplacement there. The granite fabric is less organized in the east half, corresponding to the discordant nature of the pluton contact there. They concluded that the pluton assembled by forcible intrusion of successive pulses of magma at a crustal depth of 12–15 km. Thus, initial pluton formation involved magma ascent in a vertical west- northwest–striking feeder dike, now represented by the apophysis, which was arrested at a stratigraphically controlled mechanical discontinuity in the overlying Cambrian metasedimentary rocks, leading to formation of a southwest-dipping sill. We believe that the discontinuity is the change from anhydrous to hydrous rocks that is located near the top of the Campito Formation.
The sill, accompanied by horizontal infilling from the feeder dike at the base of the sill, subsequently inflated. Law and colleagues postulated that downward cooling from the roof of the pluton prevented further vertical inflation on the west side of the pluton, so that the magma was forced expand laterally northeastward. That inflation, which deformed and vertically translated previously emplaced magma pulses and local raised the sill roof, was facilitated by thermal and hydrolytic weakening as the wall-rock temperatures progressively increased during emplacement of successive magma pulses. Field evidence of successive pulses of magma pulses is lacking, however, except for a couple minor exceptions on the northwest flank of Waucoba Mountain where contacts between subtle textural variations can be trace a few tens of meters.
Simple thermal modeling by Law and his colleagues, using microstructural and thermobarometric data, indicates that the total duration of emplacement of the pluton did not exceed 30,000 yr. They concluded that this rapid emplacement rate may explain why the pluton appears to be anorogenic even though it was emplaced during a period of regional deformation.
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