A geologic investigation of the Halloran Hills,central Mojave Desert,California |
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| Authors: | Dr D A Warnke |
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| Institution: | (1) Present address: Dept. of Oceanography, Florida State University, 32 306 Tallahassee, Florida, USA |
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| Abstract: | The Halloran Hills consist of Precambrian (?) metamorphites, Mesozoic igneous intrusive rocks, Tertiary volcanic and sedimentary breccias, and Quaternary alluvia. The Precambrian Halloran Complex has been subdivided into the following formations: (1) Silver Lake Peak Formation, mostly quartzofeldspathic gneisses; (2) Cree Camp Formation, quartzites and metarhyolites; (3) Riggs Formation, metamorphosed carbonate rocks. This complex is intruded by dioritic rocks. Regional metamorphism produced parageneses of the almandine amphibolite facies. Metablastesis was a major phenomenon, partial fusion was a local one. The rocks are, therefore, metatexites. Regional metamorphism was followed by diaphthoresis, accentuated in a zone of dislocation. A mesozonal pluton (Wander Mine Pluton) invaded the Halloran Complex during the Laramide (?) orogeny. The plutonic core consists of quartz monzonite, whereas the peripheral parts are more basic in composition. Contact relationships with the country rock are both concordant and discordant. Locally, a tectonic breccia marks marginal upthrusts. Foliation exists to various degrees in the periphery of the pluton. The sequence of the emplacement phases of the pluton is as follows: (1) metasomatism, (2) flow, and (3) movement of the dead body. All three phases overlapped each other, but at the present level of erosion the rocks in any one restricted area depict only two phases. Younger granitic rocks and various dike-swarm intrusions followed the emplacement of the pluton. During the Tertiary, autoclastic friction breccias of andesitebasalt composition intruded along faults. These intrusions were followed by the deposition of arkosic breccias (Halloran Spring Formation) and coarse sedimentary breccias (rubble). At the end of the Tertiary (Pliocene?), basalt flows buried much of the ancient land surface near Halloran Spring. The configuration of the Halloran complex indicates a domelike structure, partially bounded in the north by a fault zone (Cree Camp Fault Zone). Outlines of the Wander Mine Pluton indicate a phacolithic geometry modified by regional faults. The most important fault in the area is the Cree Camp Fault Zone, which has a general east-west trend. It was probably formed before the emplacement of the pluton, and was revived three times after its initiation. Faulting ceased after the outpouring of the basalt flows. Within the regional framework, the Halloran Hills lie east of the quartz-diorite line. There seems to be a trend for association of tonalites with sillimanite-cordierite-bearing metamorphites west of the quartz-diorite line, and of quartz monzonites with potassium-rich metamorphites east of it. If the plutonic rocks are anatexitic, or palingenetic, their compositional differences on either side of the line find a suitable explanation. Results of recent experiments agree with this assumption. “Anormal” rock suites are explained as ectectic mobilizates. The age of orogenies in California generally decreases from west to east, and the potassium-sodium ratio increases in the same direction. It is submitted that time determines the site of the orogeny and that the site determines the possible range in composition of the granitic magma. The spreading sea-floor hypothesis serves as the unifying framework for the above considerations. |
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