Proterozoic evolution of the Mojave crustal province as preserved in the Ivanpah Mountains, southeastern California
Highlights
► A prolonged tectonic history is preserved in the Ivanpah Mountains, SE California. ► Proterozoic supracrustal rocks were deposited on a continental margin. ► U–Pb ages of zircon and monazite demonstrate multiple periods of metamorphism. ► Migmatization occured at 1.67 Ga at ∼750 °C and 3.5 kb.
Introduction
Exposures of deeply exhumed rocks can provide an understanding of the formation of continents and the growth of cratons through periods of accretion. In the western United States, widespread exposures of Precambrian crust have been exhumed by Mesozoic and Cenozoic tectonic events. The signature of the processes that assemble continents is recorded in these complex middle crustal exposures that have undergone protracted or multiple events.
Distinct Proterozoic crustal provinces have been identified (e.g., Condie, 1982, Bennett and DePaolo, 1987, Wooden and DeWitt, 1991), although the order and timing of assembly of these provinces is still debated (e.g. Duebendorfer et al., 2006, Whitmeyer and Karlstrom, 2007, Amato et al., 2008). This paper shows how a detailed tectonic history can be unraveled in a key type-area using zircon and monazite geochronology, thermobarometry and stable isotope geochemistry.
From about 1.8 to 1.6 Ga, newly formed continental crust was added to southern Laurentia during a significant period of crustal growth (e.g., Condie, 1982, Karlstrom and Bowring, 1988, Hoffman, 1988, Karlstrom et al., 2001). Paleoproterozoic crust of this age forms approximately 20% of the North American craton and stretches from southern California to Nova Scotia along a NE trend (for details see Whitmeyer and Karlstrom, 2007). Geochronologic, isotopic, and structural studies have defined crustal provinces and terranes that generally young to the south/southeast with increasing distance from the Archean craton (e.g. Condie, 1982, Bennett and DePaolo, 1987, Karlstrom and Bowring, 1988, Amato et al., 2008). Much of this crust has been interpreted as juvenile island arcs and marginal basins that formed with limited involvement of pre-existing continental crust (e.g., Condie, 1982, Bennett and DePaolo, 1987, Karlstrom and Bowring, 1988, Wooden and DeWitt, 1991). However, Shufeldt et al. (2010) documented Archean and 1.8–2.0 Ga detritus in the Vishnu schist of Grand canyon, which implies that this sequence was derived, at least in part, from older crust.
The Mojave crustal province of western Arizona and southeastern California (Fig. 1) is distinct from the other Paleoproterozoic crustal provinces based and Nd and Pb isotopic studies (e.g., Bennett and DePaolo, 1987, Wooden and DeWitt, 1991, Duebendorfer et al., 2006). The Mojave province was originally defined by rocks with Nd model ages of 2.0–2.3 Ga, which is significantly older than their crystallization ages (Bennett and DePaolo, 1987). Studies of Pb isotopic compositions have shown that the Mojave crust is significantly more radiogenic when compared to the relatively juvenile Pb isotopic compositions of the adjacent Yavapai province, and that the Mojave crust has an inherently higher Th/U ratio (Wooden and Miller, 1990, Wooden and DeWitt, 1991, Barth et al., 2000, Duebendorfer et al., 2006). SHRIMP U–Pb ages of detrital zircons from Paleoproterozoic metasediments in the Mojave province have documented abundant Archean detrital zircons and inherited Archean zircons have been reported in plutonic samples from the Mojave province (Barth et al., 2000, Bryant et al., 2001, Strickland et al., 2009, Shufeldt et al., 2010). Therefore, the Mojave province, unlike other Paleoproterozoic crust such as the adjacent Yavapai province, likely incorporated a significant amount of older, pre-existing crustal material during its formation ca. 1.79–1.74 Ga (Bennett and DePaolo, 1987, Wooden and Miller, 1990, Barth et al., 2000, Duebendorfer et al., 2010).
The Ivanpah Mountains of southeastern California (Fig. 1) contain a relatively unstudied Proterozoic migmatite terrane that belongs to the Mojave crustal province. The purpose of this paper is to characterize the Paleoproterozoic rocks exposed in the Ivanpah Mountains and to determine the timing and nature of tectonic events that they preserve. Detailed U–Pb geochronology of zircon and monazite, and zircon oxygen isotope ratios measured by secondary ionization mass spectrometery (SIMS) combined with thermobarometry and field relations are combined to identify potentially four periods of metamorphism and/or magmatism between 1.76 and 1.67 Ga. The goals of this paper are (1) to establish the ages preserved in the rocks of the Ivanpah Mountains and try to relate them to fabrics and tectonic events, (2) to compare and contrast zircon and monazite formation in multiply-metamorphosed rocks, (3) to use oxygen isotopes to determine the nature of zircon growth, (4) to use in situ analytical techniques to determine the formation of the penetrative migmatitic fabric, and (5) to use thermobarometry to determine the pressure and temperature of the migmatization. This study will provide a detailed fingerprint of the crust of the Mojave province that is necessary for paleocontinent reconstructions, and will help to constrain the growth of continental crust along the southern margin of Laurentia during the Paleoproterozoic. We demonstrate that the timing of migmatite formation in the Ivanpah Mountains was likely ca. 1.67 Ga, and may have occurred in an extensional setting.
Section snippets
Geologic setting
Exposures of Paleoproterozoic basement in the southwestern United States preserve a record of crust formation and amalgamation along a long-lived convergent margin (e.g., Karlstrom and Bowring, 1988). In the Mojave province, the oldest known rock is the 1.84 Ga Elves Chasm tonalite gneiss (Hawkins et al., 1996, Ilg et al., 1996), but due to the limited exposure in the Granite Gorge of the Grand Canyon its tectonic significance is unclear. Whitmeyer and Karlstrom (2007) interpreted the Elves
The Ivanpah Mountains
The Ivanpah Mountains are located in the central Mojave province more than 100 km to the west of the isotopically mixed boundary with the Yavapai province, and can be viewed as a type-location for Mojave crust (Wooden and Miller, 1990, Duebendorfer et al., 2006). In the eastern Ivanpah Mountains of southeastern California (Fig. 1), exposures of Proterozoic crystalline basement are characterized by series of banded gneisses of both igneous and sedimentary origin. These rocks were transposed into
Methods
Samples collected from the Ivanpah Mountains were chosen to reflect a variety of rock types and degrees of deformation for the purpose of identifying multiple periods of deformation and metamorphism. U–Pb ages of zircon and monazite were determined using the U.S.G.S./Stanford University SHRIMP-RG in multiple sessions. Data reduction follows the methods described by Williams (1997) and Ireland and Williams (2003) and uses the Squid and Isoplot programs of Ken Ludwig. For all zircon samples,
Metagabbro
Samples IV1 and IV5 are from metagabbro exposed in the eastern Ivanpah Mountains (Fig. 1; Table 1). The metagabbro is coarse grained, granoblastic to porphyritic with large plagioclase crystals in a matrix of equigranular clinopyroxene and orthopyroxene. Cathodoluminescence images of zircons from the metagabbro show irregular, embayed grains with oscillatory zonation and thin, low-U overgrowths (Fig. 3). Analyses of the zircon interiors from the two samples produced concordia intercept ages of
Detrital zircons from paragneisses
Detrital zircons are abundant in the paragneisses from the Ivanpah Mountains. Zircons from the paragneisses are typically comprised of oscillatory zoned interiors mantled by narrow, unzoned overgrowths (Fig. 3). Analyses of zircon cores yield a distinctive detrital age population, and Fig. 7 shows a histogram of 207Pb/206Pb ages from 180 zircon interiors from five samples of paragneiss. Determination of the youngest detrital grain from each sample (Table 2) was based on 232Th/238U values, and
Thermobarometry
Spectacular, large garnets are ubiquitous in the garnet–biotite gneisses (Fig. 2). Many are euhedral to subhedral and inclusion-rich. Large quartz inclusions are common, and the garnets also contain inclusions of plagioclase, K-feldspar, biotite, sillimanite, monazite and zircon. Interestingly, no inclusions of cordierite were identified in garnet although cordierite is abundant in the matrix assemblage. The garnets are almandine-rich and nearly homogeneous in major element composition (
Oxygen isotopes in zircon
Oxygen isotope ratios were determined from the same grains that were dated by U–Pb analysis by using the IMS-1280 housed at the University of Wisconsin. The oxygen analysis pits were placed adjacent to U–Pb pits in the same CL zone (see Appendices A and 5). Detrital zircons in the metasedimentary rocks show a significant difference between the two main detrital age populations (Fig. 7, Fig. 14). The Archean cores have δ18O values of 6.3 ± 1.4‰ (n = 14) (Fig. 14), which is consistent with δ18O of
Zircon vs. monazite behavior
Zircons and monazites from the paragneisses exposed in the Ivanpah Mountains record multiple periods of metamorphic growth, but the patterns of zircon ages differ between samples (Table 4). Several possibilities may explain the variation of zircon ages: (1) these zircons preserve a record of prolonged or repeated metamorphic growth during tectonic events from ca. 1.76 to 1.65 Ga, (2) the formation of metamorphic overgrowths on zircon first occurred at 1.74 Ga and was then variably overprinted by
Conclusions
Banded gneisses from the Ivanpah Mountains record over 100 million years of tectonic events that occurred during the Paleoproterozoic. The oldest rocks are paragneisses made up of immature metasediments that were deposited ca. 1.79–1.76 Ga, and intruded by gabbro, tonalite, and granite at 1.76 Ga. These units likely formed in an arc setting on or adjacent to a continent. Metamorphism, deformation and partial melting occurred initially at 1.74 Ga. Evidence for metamorphism during the 1.70 Ga Ivanpah
Acknowledgments
We would like to acknowledge several people who made this research possible. We thank Elizabeth Miller for her support, revisions and numerous discussions of this manuscript. We also thank Andy Barth for helpful discussions. Mike Spicuzza aided in the generation of the laser fluorination data, and John Fournelle assisted with the electron microprobe data. We than Brad Ito for his assistance with the SHRIMP-RG, and Noriko Kita and Jim Kern for their assistance on the IMS-1280. We also thank
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