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K-Ar Thermochronology of a Mesozoic Plutonic Complex, Avawatz Mountains, Southeastern California J.E

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K-Ar Thermochronology of a Mesozoic Plutonic Complex, Avawatz Mountains, Southeastern California J.E K-Ar thermochronology of a Mesozoic plutonic complex, Avawatz mountains, southeastern California J.E. Spencer Isochron/West, Bulletin of Isotopic Geochronology, v. 48, pp. 3-8 Downloaded from: https://geoinfo.nmt.edu/publications/periodicals/isochronwest/home.cfml?Issue=48 Isochron/West was published at irregular intervals from 1971 to 1996. The journal was patterned after the journal Radiocarbon and covered isotopic age-dating (except carbon-14) on rocks and minerals from the Western Hemisphere. Initially, the geographic scope of papers was restricted to the western half of the United States, but was later expanded. The journal was sponsored and staffed by the New Mexico Bureau of Mines (now Geology) & Mineral Resources and the Nevada Bureau of Mines & Geology. All back-issue papers are available for free: https://geoinfo.nmt.edu/publications/periodicals/isochronwest This page is intentionally left blank to maintain order of facing pages. K-Ar THERMOCHRONOLOGY OF A MESOZOIC PLUTONIC COMPLEX, AVAWATZ MOUNTAINS, SOUTHEASTERN CALIFORNIA JON E. SPENCER Arizona Bureau of Geology and Mineral Technology, Tucson, AZ 85719 The Avawatz Mountains are located at the southern end types dated, the quartz monzonite of Cave Spring Wash of Death Valley In the northeastern Mojave Desert region of contains one to several percent pyroxene which could not southeastern California. The eighteen conventional K-Ar be completely separated from the hornblende, raising the dates reported here reveals long and complex cooling possibility that excess argon trapped in pyroxene resulted history. in an anomalously old age. GEOLOGY. The crystalline rocks forming most of the Younger ages were obtained from all other samples. Ex arcuate Avawatz Mountains are exposed within an cluding the 174.7 m.y. hornblende age (AV-1269), the elongate, northwest-trending fault block bounded to the 1 54.1 m.y. hornblende age (AV-961), and the young ages northeast by the active Mule Spring and Mormon Spring from samples in the far northwestern Avawatz Mountains reverse faults and to the southwest by the Miocene and the west edge of the map area, the 5 other hornblende Arrastre Spring fault (fig. 1). Diachronous vertical Tertiary ages fall in the range of 141.6 to 91.1 m.y. Excluding the movement on both faults has resulted in uplift and ex young biotite age from the far northwestern Avawatz posure of a Plutonic complex that form most of the range Mountains, all biotite ages (7 ages) fall in the range of (Spencer, 1981). 1 38.8 to 66.6 m.y. These ages span a time period during which most of the exposed crystalline rocks cooled from Five Plutonic rock types are exposed within the Avawatz greater than approximately 500°C to less than approxi Mountains. The Avawatz quartz monzodiorite, by far the mately 300°C (argon closure temperatures from Harrison, most areally extensive rock type, is generally rnedium 1981, and Harrison and others, 1985). This long, gradual grained, equigranular, and contains up to 40% biotite and cooling history is interpreted to have resulted during ero- hornblende. It has been dated at 175 ± 5 m.y. by the U/Pb sional denudation and uplift, perhaps punctuated by heat method (DeWitt and others, 1984). The older granite of pulses associated with plutonism and more rapid cooling Avawatz Peak forms pendants and inclusions within the due to tectonic uplift. The small discordance between Avawatz quartz monzodiorite along the crest of the range biotite and hornblende ages in two samples (AV-132 and and is exposed on the southwest side of the Arrastre AV-133) indicates rapid cooling probably related to local Springs fault where it intrudes Mesozoic ® magmatism or uplift due to tectonism. Emplacement of metafedimentary rocks. P®"dants of pre-Mes^o.c m^ fine-grained mafic dikes in the vicinity of sample AV-1 32 sedimentary rocks are also exposed at higher elevations in could have caused heating and argon loss. The wide the range The Avawatz quartz monzodiorite is intruded by scatter of the K-Ar ages conceal any regional heat pulses Se grarodiorfte of Mormon Canyon in the central part of related to magmatism or widespread rapid cooling related the range The quartz monzonite of Cave Spring Wash in to tectonism. the northwestern Avawatz Mountains is faulted 393'"^ The Avawatz quartz monzodiorite yielded relatively the Avawatz quartz monzodiorite; the relative ages young ages in the far northwestern Avawatz Mountains. The biotite age of 45.7 m.y. is surprisingly young consider 'The CoriE^fold and thrust belt is well represented to ing that early Tertiary igneous rocks are virtually nonexist ent in the Mohave region (Snyder and others, 1 976; Coney and Reynolds, 1977). The quartz monzodiorite has a strong, south-dipping crystalloblastic foliation in this area which dies out eastward over a distance of 4 kilometers thrusts. The Ava , batholith belt of western Pegmatite dikes, both discordant and concordant to folia eastern edge /'^qjonal relationships suggest that tion, are abundant in the strongly foliated rock, and are in North America. T , Avawatz Mountains was a conse- terpreted as broadly synchronous with the foliation cooling of rocks in Ava following batholith emplace- producing deformation (Spencer, 1981). The 62.4 my quence of reQ'onal heat loss follow g ^ age on coarse muscovite from a post-deformation pegrna- ment plus thickening. tite places a younger limit on the age of deformation. The 45.7 m.y. biotite age is interpreted as either: (1) recordinq ''I.SsStoNDISCUSbiuw. The1!"P'=. M74.7 ±1r"r"•1 m.y.) was col- early Tertiary cooling during gradual, regional, earlv hornblende date o considered to be the structurally Cenozoic denudation and uplift, following regional lectedfromanar complex, from just below a Mesozoic magmatism; or (2) recording rapid cooling durino highest part o ^ ,^,^^31 contact at the base of part tectonic uplift of a fault sliver along an early Tertiarv middle Miocene deposi lA/ithln the belt of ore- ancestral Garlock fault. The presence of lower Tertiarv of the Avawatz Formation, and within the oeit or pre KM ^ „^riants and older Mesozoic granites t sedimentary rocks along the western Garlock fault and ™s is sssemiallv idsmlcsl to their virtual absence elsewhere in the Mohave region is thri/Pb age of the Avawatz quartz monzodiorite reported evidence supporting the existence of an ancestral Garlork bv DeWitt and others(1984), and the sample is interpreted fault (Dibblee, 1967; Nilsen and Clarke, 1975; Cox to have remained below the closure temperature for argon 1982) unrelated to Miocene and younger extensional tec' in hornblende since crystallization. Biotite from the same tonism (Davis and Burchfiei, 1973). sample yielded the oldest biotite age (138.8 ± 0.8 m.y.). The 77.6 m.y. hornblende age from a dioriticpluton that The second-oldest hornblende age (1 54.1 m.y.), from a IS thrust over Mesozoic volcanic and sedimentary rocks sample (AV-961) of the quartz monzonite of Cave Spring west of the Avawatz Mountains, possibly represents Wash, is considered suspect because, unlike the other rock either: (1) the crystallization age of the pluton; or (2) uplift [ISOCHRON/WEST, no. 48, April 1987] 4 AV-145 AV-172 116^30' 62.4 M 45.7 B 76.8 H AV-133 86.3 B 91.1 H AV-961 135.1 B 15154.1 H >4^ Fi-e 77.6 H ?- »" ^ V.-:':V- {• •/:*• •• *. ••: A A A - V m ••••# m •A • •_ •_• • _ • SURFICIAL DEPOSITS (QUATERNARY) SEDIMENTARY AND VOLCANIC ROCKS (TERTIARY, LOCALLY INCLUDES PLEISTOCENE DEPOSITS) ♦ ♦ ♦ GRANODIORITE OF MORMON CANYON ♦ ♦ ♦ 7. t t t . (JURASSIC OR CRETACEOUS) AVAWATZ QUARTZ MONZODIORITE (JURASSIC) k %»*,4 GRANITE OF AVAWATZ PEAK * a* %■% , U4.4.4.4.-I. QUARTZ MONZONITE LL±3u (JURASSIC OR TRIASSIC) ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ OF CAVE SPRING WASH It t ♦ » (MESOZOIC) METAVOLCANIC AND METASEDIMENTARY />'»*•*»**»» •! ROCKS (MESOZOIC) GRANITIC ROCKS, METASEDIMENTARY ROCKS UNDIVIDED (MESOZOIC) (PALEOZOIC AND PRECAMBRIAN) CRYSTALLINE ROCKS (PRECAMBRIAN) FIGURE 1. Simplified geoiogic map of the Avawatz Mountains and adjacent areas to the west. K-Ar sample locations and ages are also shown. Geologic map data from (1) Brady (1986), (2) Spencer (1981), (3) Troxeland Butler (1979), and (4) Troxel, (1979). [ISOCHRON/WEST, no. 48, April 1987] iie-is AREA CALIF. AV-132 119.6 B 112.7 H 35'35' AV-1070 66.6 B I06.7H » t• #• • • * •• • •• ••••••• 35''30 - 141.6 H AV-338 82.5 B 138.88 128.0 H ipjO ,;o^;rgao.Y^o9^^^ V A • • .O •• SOURCES MAP DATA SqP" CyT; ftTtwi [ISOCHRON/WEST, no. 48, April 1987] and cooling during and immediately after latest-Cretaceous 40Ar»/^oAr = 91.30/0; (hornblende) o/oKzO = 1.456, thrusting. 1.459; ^oAr» = 1.96038 x 1 Q-io mol/g; ^0Ar*/^0Ar CONCLUSIONS. Conventional K-Ar thermochronology = 53.8%. of the Mesozoic plutonic complex reveals a complex cool (biotite) 86.3 ± 0.5 m.y. ing history. Most of the K-Ar ages were set during (hornbiende) 91.1 ± 0.6 m.y. Cretaceous time. The wide scatter in K-Ar ages is sug gestive of gradual regional uplift and erosional denudation 3. AV-145 K-Ar as the cause of cooling through argon-closure-temperature Pegmatite dike (1 1 6°30'53" W,35°36'1 0" N; CA) isotherms, although magmatism and tectonism probably in strongly foliated Avawatz quartz-monzodiorite. played an important role, at least locally. Analytical data: %K20 = 10.75, 10.77; ^^^Ar* = 9.83358 X 10-10 mol/g; ^oAr»/^oAr = 91.5%. Co/n- ACKNOWLEDGMENTS ment: pegmatite dike is unfoliated and cuts across foliation at a high angle. This report represents part of a Ph.D. dissertation done (muscovite) 62.4 ± 0.4 m.y. under the supervision of Prof. B. C. Burchfiel at the Massachusetts Institute of Technology.
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