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Eruptive History and Structural Development of the Toquima Caldera Complex, Central Nevada

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Eruptive History and Structural Development of the Toquima Caldera Complex, Central Nevada Eruptive history and structural development of the Toquima caldera complex, central Nevada DAVID R. BODEN Department of Geology, Stanford University, Stanford, California 94305 ABSTRACT The Toquima caldera complex, located in the Toquima Range of central Nevada, consists of three overlapping to nested calderas. The Moores Creek caldera is the largest (~30 by 20 km); it formed -27.2 Ma in response to eruption of the high-silica rhyolite tuff of Moores Creek. Because of recurrent volcanic activity and subsequent basin- range faulting, only the northern segment of the Moores Creek caldera is preserved; its eastern and western margins are downfaulted below valley fill, and its southern part was obscured by collapse of the Mount Jefferson caldera. Eruption of the tuff of Mount Jefferson resulted in collapse of the 18- by 20-km Mount Jefferson caldera [ Silicic Ignihibrites -26.5 Ma. The ash-flow tuff exposed at Round Mountain is a silicic -J Austin' outflow-facies equivalent of the compositionally zoned (76-67 wt % Other Volcanic Roqks Si02) intracaldera tuff of Mount Jefferson. Pyroclastic eruptive activ- ity in the complex concluded -23.6 Ma with formation of the compar- / <f atively small 8- by 10-km Trail Canyon caldera. With time, caldera / O / / / ©^ f\ size diminished, and the focus of volcanic activity shifted progressively / . southeastward. The southeastward migration of volcanism in the <t • • Ul • complex approximately parallels regional northwest-striking faults, \ NC . O \ suggesting fundamental structural control in the rise and eruption of F 9 2 magma. / vi ; - D Arcuate faults and small, aphyric to porphyritic plugs outline the ( Vi structural margins of the calderas. Some plugs, on the basis of grada- tional textural changes from vitroclastic in the tuff to flow-layered and porphyro-aphanitic in the plug, appear to represent lava-choked, ash- flow-tuff, feeder vents. Caldera collapse breccias are locally well ex- posed in all three calderas. Caldera resurgence is not strongly developed in the Toquima caldera complex. The Mount Jefferson massif, which dominates the complex at an elevation of 3,640 m, is an artifact of young basin-range block faulting. The limited development of resurgence in the Mount Jefferson caldera is attributed to ponding of the bulk of erupted mate- rial within the caldera. Intermediate to mafic volcanic rocks, premonitory or coeval to silicic ash-flow tuff, are volumetrically minor over central Nevada and are lacking in the Toquima caldera complex. The less silicic magma apparently became lodged in the middle to lower crust below an extensive zone of more silicic, less dense magma. INTRODUCTION Figure 1. Regional location map of the Toquima caldera complex, showing outlines of the individual eruptive centers. Other major geo- Voluminous and areally extensive mid-Tertiary ash-flow tuff covers logic features shown include the Northumberland and Manhattan cal- much of central Nevada, but the location, configuration, and evolution of deras (NC, MC, respectively) and the Toiyabe-Kawich lineament most source areas remain dimly understood. This is because detailed geo- (TKL). Upper box shows the location of the enlarged area with re- logic studies in the region are few, eruptive centers have been obscured by spect to the west-northwest-trending belt of Oligocene to early Mio- recurrent volcanic activity, and late Tertiary and Quaternary basin-range cene volcanic rocks, dominated by silicic ash-flow tuff. MVF denotes faulting has disrupted and truncated parts of the source areas. the Marysvale volcanic field. Geological Society of America Bulletin, v. 97, p. 61-74, 9 figs., 2 tables, January 1986. 61 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/61/3445131/i0016-7606-97-1-61.pdf by guest on 25 September 2021 62 D. R. BODEN Source Intracaldera Extracaldera Intrusions Tre j i- . Trail Canyon MSI i * <• i * caldera Tt yp Ttb Mt.Jefferson ISP caldera Tju STjr, +++++++4 , Contact Moores Cree^k caldera f+Tmc V» + + + Fault: ball on downdropped side; dashed where approximately located. ^,-oTcc Cauldron boundary: ball on downthrown ». ;.o ; side of margins reactivated by basin Unknown it ; • ^-range faulting: rectangle on • Tis." caldera side. calderas •jO^ Strike and dip of foliation '•Tdb - Horizontal eutaxitic foliation Qd Alluvium,Colluvium,and Fanglomerate H4" Location of K-Ar sample (Table 2) •-t Topographic wall "TrcJ, tuffs of RoadCanyon (23-22Ma) tuff of Trail Cyn (23.6 Ma) Ttl-local capping porphyritic lava ¿TdPÍ» megabreccia of Dry Canyon (32.3 Ma) Tt-upper and lower members undifferentiated Ttb-caldera collapse breccia Porphyritic toaphyric plugs and llava-chocked ignimbrite feeder vents tuff of Ryecroft Canyon (25.0 Ma) tuff of Mt. Jefferson (26.4 Ma) V V s / Granitic rocks of Shoshone Mountain isTjCÈ^Tic— discontinuous capping cooling units Kg's •Tju- Tju- upper member WiFTI Tjl- lower member: solid triangles denote caldera fi'f ' j :ollapse breccias Pzs; lower Paleozoic sedimentary , | ; J Tjr- outflow Round Mountain member (26.7 Ma) rocks M Pzb; severely brecciated Paleozoic + +4 + + y rocks and megabreccia. tuff of Moores Creek (27.2 Ma) its own story to tell, involving not only caldera-related processes, but regional lithospheric magmatism and tectonism, as well. tuff of Corcoran Canyon (27.5 Ma) This report chronicles the eruptive history and structural develop- ment of the Toquima caldera complex—a major ignimbrite source area located in northern Nye County, Nevada, -400 km east of Reno (Fig. 1). tuff of LoganSpring (29.6 Ma) PK8 The complex is situated in the Toquima Range and consists of three overlapping to nested calderas of Moores Creek (27.2 Ma), Mount Jeffer- The development of calderas reflects a complex feedback between son (26.5 Ma), and Trail Canyon (23.6 Ma) (Fig. 2). The town and Au-Ag magmatic and tectonic processes. Their evolution is influenced by the mine of Round Mountain are located in the southwest corner of the study pre-existing structural fabric of the basement, state of crustal stress at the area. Data for this summary are derived from detailed mapping (1:24,000 time of development, thermal and mass flux through the lithosphere, and scale), K-Ar dating, petrography, and selected whole-rock chemical depth and residence time of magma prior to eruption. As a result, each has analyses. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/61/3445131/i0016-7606-97-1-61.pdf by guest on 25 September 2021 Figure 2. Generalized geologic map of the Toquima caldera complex. Inset shows the inferred configurations of the nested Moores Creek (MC), Mount Jefferson (MJ), and Trail Canyon (TC) calderas prior to the onset of basin-range block faulting. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/1/61/3445131/i0016-7606-97-1-61.pdf by guest on 25 September 2021 64 D. R. BODEN TABLE 1. CALDERAS AND RELATED ROCK UNITS OF THE TOQUIMA PREVIOUS WORK CALDERA COMPLEX, CENTRAL NEVADA Early work in and around the Toquima caldera complex concen- Caldera Eruptive Unit Age SiOjStt Volume 3 trated on describing the geology and mineralization of various mines, (m.y.)' (km ) including Round Mountain (Ransome, 1909; Ferguson, 1921) and Jeffer- Moores Creek Tuff of Moores Creek 23.6 ± 0.4 76.0-72.5 X« son Canyon and Gold Hill (Krai, 1951). Ferguson and Cathcart (1954) j ' Tuff of Mount Jefferson published a geologic map of the 30-minute Round Mountain quadrangle. Mount Jefferson 1 Upper member 1 intra. 26.4 ± 0.5 75.7-67.7 -250 1 Lower member f caldera Kleinhampl and Ziony (1985) reviewed the geology and compiled a re- 1i Round Mountain mbr 26.7 ± 0.6 77.5-73.8 >10 connaissance geologic map of northern Nye County that depicts general Trail Canyon Tuff of Trail Canyon 27.2 ± 0.6 78.0-76.8 >(00 rock relations wi:hin the Toquima caldera complex. A generalized geo- logic map of the Northumberland caldera (McKee, 1974) includes the •Weighted mean iige . * XRF bulk tuff analyses. northern half of tlie Toquima caldera complex. Marvin and others (1973) and Silberman and others (1975) published K-Ar ages on rock samples collected from Mount Jefferson and Round Mountain, respectively. More recently, Tingley and Berger (1985) reported on the geology meso- and megabreccias (terminology after Lipman, 1976a). Small, and geochemistry of lode deposits in the Round Mountain Au-Ag mine. aphyric to porphyritic, flow-layered, intrusive plugs occur locally. Bedded, Detailed geologic: relations in the vicinity of the mine are depicted in the tuffaceous sedimentary rocks are relatively minor, and airfall tv.ffs and 7.5 minute Round Mountain quadrangle (Shawe, 1981a). Results of a lavas are surprisingly scarce. regional gravity survey by Snyder and Healey (1983) show the Toquima Eight major volcanic units, ranging in age from 32 to 22 m.y., are caldera complex to be outlined by a multilobed gravity low. Harrington exposed in and around the Toquima caldera complex (Fig. 2). Three (1983) reported on the geology of a part of the Mount Jefferson caldera. units—the tuffs of Moores Creek, Mount Jefferson, and Trail Canyon— Detailed investigations of alteration and mineralization in the active were erupted from the complex (Table 1). Other map units include the open-pit mine at Round Mountain are reported by Mills (1984), Sander megabreccia of Dry Canyon and tuffs of Logan Spring, Corcoran (Canyon, and Mills (1984), and Mills and Sander (in press). Ryecroft Canyon, and Road Canyon, all having their sources elsewhere. The 32-m.y.-old megabreccia of Dry Canyon is exposed just north- GEOLOGIC S ETTING east of Round Mountain (Fig. 2). It is inferred to mark the eastern fringe of a caldera that lies largely buried by valley fill to the west. The 29.5-m.y.- The Toquima caldera complex lies near the middle of an extensive old tuff of Logan Spring is exposed only in a small area in upper Road west-northwest-trending belt of Oligocene and early Miocene volcanic Canyon; its source is unknown, and it is unclear whether the tuff is an rocks, consisting mainly of silicic ash-flow tuff (Stewart and others, 1977; outflow or intracaldera accumulation.
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