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The intrusive history of the Little Chief granite porpyhry stock, central Panamint Range, California : I. Structural relationships. II. Petrogenesis, based on electron microprobe analyses of the feldspars


McDowell, Stewart Douglas (1967) The intrusive history of the Little Chief granite porpyhry stock, central Panamint Range, California : I. Structural relationships. II. Petrogenesis, based on electron microprobe analyses of the feldspars. Dissertation (Ph.D.), California Institute of Technology.


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The Little Chief stock of Upper Miocene (?) age, located in the central Panamint Range near Death Valley, California, crops out over an area of 2.5 by 4.5 miles and through an elevation range of 6300 feet. It is a crosscutting diapir-like body with contact attitudes ranging from vertical to 80° outward except along the eastern margin, where the contact attitudes range from 45° outward to 35° inward. The stock appears to neck downward into a thin, east-west, dike-like body at a depth of less than 6000 feet below the present surface.

The stock has lifted a nearly rectangular "trapdoor" of later Precambrian and Precambrian (?) rocks bounded by vertical faults on the north, west, and south. The trapdoor opens to the west with a vertical throw of 3000 feet on the western margin and with a roughly north-south hinge line located just east of the stock. East of the hinge line, the trapdoor has been slightly depressed. The trapdoor offsets an earlier set of westward-dipping normal faults along which a porphyry dike swarm has been intruded.

The stock consists of two intrusive phases, a south and a slightly later north phase. The stock magma moved to a position roughly 6000 feet below the present surface and formed the westward-dipping normal faults by stretching the sedimentary rocks over the magma chamber roof, which were immediately injected by the porphyry dikes. The magma moved to the present level, truncating the normal faults and associated dikes, forming the trap door, and doming the sediments of the roof. The interior of the north phase then moved slightly upward to the present level, disrupting the eastern part of the trapdoor and renewing movement on some of the trapdoor faults.

The stock is a hornblende-biotite granite porphyry with 2-10mm normally-zoned sanidine and plagioclase in a quartz-alkali feldspar-plagioclase groundmass. Detailed electron microprobe analyses yield the following feldspar compositions. The plagioclases have distinct normally-zoned layers separated by abrupt compositional gaps. The gaps formed by sporadic, abrupt water pressure decreases as the magma moved upward. Assimilation of water-rich calcic material caused large calcic oscillations on the plagioclase zones. Zonation in the outermost edges of the plagioclase phenocrysts to very low-Or plagioclase could be due to [...] increase and/or the effect of the peristerite solvus on the ternary feldspar solvus.

Sanidine coexisted with [...] plagioclase in a magma chamber 12,000 feet below the present level, which had an undifferentiated, unsaturated [...] core surrounded by a differentiated, saturated [...] exterior. This [...] variation was equalized as the magma moved up to its present position, where the groundmass crystallized at [...] some 7000 feet below the contemperaneous ground surface.

Sanidine was removed from equilibrium, due to its rapid crystallization and shift of the feldspar boundary on [...] increase, at different times. It was rimmed by [...] plagioclase in the water-poor core of the magma chamber before the magma started to move upward. Toward the exterior sanidine began to be replaced by sodic oligoclase as the magma moved upward, and in the water-rich exterior parts sanidine exsolved to a patch perthite while the system was 50 percent melt, and the K-phase was then replaced by sodic oligoclase. In the very water-rich parts, sanidine crystallized in equilibrium until the groundmass was formed.

During upward movement assimilation of dolomitic rock formed a contaminated marginal phase which was then intruded by uncontaminated magma and carried upward as inclusions.

Fracturing of the roof of the present stock led to volatile concentration in certain restricted portions of the stock, and formation of quartz macrocyrsts, graphic textures, pegmatitic pods, coarse lamellar perthite in the sanidine, and numerous vugs.

Item Type:Thesis (Dissertation (Ph.D.))
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geological and Planetary Sciences
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Albee, Arden Leroy
Thesis Committee:
  • Albee, Arden Leroy (chair)
  • Allen, Clarence R.
  • Silver, Leon T.
Defense Date:19 September 1966
Record Number:CaltechETD:etd-09252007-084230
Persistent URL:
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:3762
Deposited By: Imported from ETD-db
Deposited On:25 Sep 2007
Last Modified:26 Dec 2012 03:02

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