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Contributions of heat and/or mass from mafic magmas are commonly invoked in models of voluminous granodiorite and andesite generation in magmatic and volcanic arcs worldwide. However, mafic intrusions are a volumetrically minor component in most arc batholiths. This is the case in the Sierra Nevada batholith, California, USA, where gabbro and diorite plutons are smaller and less abundant than the granitoid suites that make up the bulk of the batholith. Here, we constrain the timing and geochemistry of mafic intrusions in the Sierra Nevada batholith to assess the role of these compositions in arc batholith construction. Previous detailed studies on a limited number of mafic intrusions demonstrate that they formed penecontemporaneously with the felsic batholith, but there is a need for a broader survey of mafic plutons using modern geochronological techniques. New U-Pb zircon ages for 13 gabbro to diorite plutons and geochemistry from 17 mafic intrusions in the eastern Sierra Nevada batholith document two main episodes of mafic magmatism in the eastern Sierra Nevada batholith, from 168 Ma to 145 Ma and from 100 Ma to 89 Ma. These episodes overlap with the ages of granitoid plutons in the eastern Sierra Nevada batholith, including the Late Jurassic Palisade Crest and Late Cretaceous John Muir intrusive suites, in addition to other felsic plutons dated in the eastern Sierra Nevada batholith. Non-primitive mineral compositions in the mafic bodies indicate that their parental magmas are the evolved products of mantle-derived basalts that first differentiated in the lower crust prior to ascent and crystallization in the upper crust. The presence of rocks with cumulate textures, as well as a wide range of bulk-rock compositions (SiO2 wt% 38−64, Mg# 39−74), show that magmatic differentiation continued within each mafic body after intrusion into the upper crust. Sr/Y ratios in melt-like (i.e., non-cumulate) mafic samples suggest that the crustal thickness of the Sierra Nevada batholith was roughly 30 km in the Early Jurassic and increased to ∼44 km by the Late Cretaceous. Concomitant intrusion of mafic melts along with voluminous granitoid plutons supports mantle melting as a major contributor of heat and magmatic volumes to the crust during construction of the eastern Sierra Nevada batholith.
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Enigmatic granites of the mid-Cretaceous magmatic flare up in the Sierra Nevada batholith: new data and insights
The Sierra Nevada Batholith (SNB) records copious Mesozoic magmatism and is an important touchstone for understanding crustal growth at continental convergent margins. Recent research in the SNB has focused on defining magmatic cyclicity and arc “flare ups” based on the ages, magma production rates, and radiogenic isotope heterogeneities of the plutonic and volcanic rocks found throughout the batholith. Two main intervals at ca. 170–148 Ma and ca. 125–85 Ma delivered >95% of the magmas in the exposed plutonic bulk in the SNB and suggest elevated emplacement rates and hotter-than-usual magmas, though the Cretaceous is by far the most productive era and the most promising for understanding the factors modulating magmatic flux. The mid-Cretaceous of the Sierra (ca. 105–98 Ma) saw the appearance of conspicuous, high-silica (>65 wt.% SiO2; average ~71%) granitic plutons of similar chemical nature that span a large geographic area, breaking the well-established west-to-east “younging” trend found in the more common rocks of intermediate compositions. This study focuses on thirteen of these high-silica granites: the Bullfrog, Independence, McGann, Rawson Creek, and Spook Plutons of the eastern Sierra; and the Shaver Intrusive Suite, Grant Grove, Case Mountain, Coyote Pass, Dennison Peak, and Frys Point Plutons of the western/central Sierra. Whole rock geochemistry, zircon trace elements, and radiogenic isotope ratios (Sr and Nd) in these high-silica granites show some transitional patterns with other contemporaneous and geographically related plutons of intermediate compositions, suggesting fractionation trajectories; however, some distinct dissimilarities are observed, including: 1) elevated, but highly varied initial 87Sr/86Sr ratios, 2) elevated fluorine in granites, and 3) hotter apparent zircon saturation conditions. These geochemical data, hotter conditions, and higher flux suggest that mantle conditions favored more crustal melting and crustal source input than at any other time in the Cretaceous. We conclude that the granitic outburst of the mid-Cretaceous was a flare up like no other.
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- PAR ID:
- 10514001
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Geological Society of America Abstracts with Programs
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract We explore the growth of lower-continental crust by examining the root of the Southern California Batholith, an ~500-km-long, paleo-arc segment of the Mesozoic California arc that lies between the southern Sierra Nevada Batholith and northern Peninsular Ranges Batholith. We focus on the Cucamonga and San Antonio terranes located in the eastern San Gabriel Mountains where the deep root of the Mesozoic arc is exhumed by the Quaternary Cucamonga thrust fault. This lower- to mid-crustal cross section of the arc allows us to investigate (1) the timing and rates of Mesozoic arc construction, (2) mechanisms of sediment incorporation into the lower crust, and (3) the interplay between mantle input and crustal recycling during arc magmatic surges. We use U-Pb detrital zircon geochronology of four quartzites and one metatexite migmatite to investigate the origin of the lower-crustal Cucamonga metasedimentary sequence, and U-Pb zircon petrochronology of 26 orthogneisses to establish the timing of arc magmatism and granulite-facies metamorphism. We find that the Cucamonga metasedimentary sequence shares broad similarities to Sur Series metasedimentary rocks in the Salinia terrane, suggesting that both were deposited in a late Paleozoic to early Mesozoic forearc or intra-arc basin marginal to the Southern California Batholith. This basin was progressively underthrust beneath the arc during the Middle Jurassic to Late Cretaceous and was metamorphosed during two high-grade (>750 °C), metamorphic events at ca. 124 Ma and 89–75 Ma. These metamorphic events were associated with 100 m.y. of arc magmatism that lasted from 175 Ma to 75 Ma and culminated in a magmatic surge from ca. 90 Ma to 75 Ma. Field observations and petrochronology analyses indicate that partial melting of the underthrust Cucamonga metasedimentary rocks was triggered by the emplacement of voluminous, mid-crustal tonalites and granodiorites. Partial melting of the metasedimentary rocks played a subsidiary role relative to mantle input in driving the Late Cretaceous magmatic flare-up event.more » « less
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Nearly two decades since the first oxygen isotope (δ18O) studies of zircon in the Sierra Nevada Batholith, California, USA, a far more extensive picture of spatial and temporal patterns of magmatic δ18O has emerged in parallel with a tenfold increase in geochronologic coverage, and many new radiogenic isotope (Sr, Nd, Hf) analyses. Over this time, models of Cordilleran-type arc systems have sought to elucidate flare-ups of magmatism as cyclic, with radiogenic isotope “excursions” tracing variable input of crust and mantle into arc magmas [e.g., 1]. Such models haven't incorporated oxygen isotopes to full advantage because of apparent complexity in the signals they record [2]. New, single zircon δ18O analyses—of plutonic, volcanic, and detrital zircon—from the Sierra amplifiing the findings of previous studies [e.g., 3], that δ18O records are well-suited for detecting relatively fast (<10 million year) recycling of subducted supracrustal rock and accreted terranes in forearc settings. Such recycling is not resolved by radiogenic isotope systems. A wealth of new volcanic δ18O zircon data from the Sierra, along with δ18O of hydrothermal minerals like skarn garnet, also records periods of significant δ18O “pull-downs” as lower-δ18O hydrothermal waters alter surface rocks whose assimilation subsequently embeds these surface signals in silicic volcanic systems. Such re-melting and volcanic episodes are often brief (< 5 million years) and small volume, so have often been overlooked, however such, δ18O values may be key to detecting plutonic from volcanic zircon in detrital records when used in conjunction with trace elements. Low-δ18O domains are becoming recognized in other arcs and to be useful to detect episodic resampling of crustal domains [4]. Morover, discovery of fossil low-δ18O systems in screens of wallrock in mid-crustal levels [e.g., 5] documents wholesale rapid burial of these domains in arcs, during transitions to episodes of shortening or transpression. All together, zircon δ18O uniquely traces surface- to-source transport and recycling in Cordilleran arcs as it relates to changing arc stress regime, at periods that may fail to be recorded in excursions of radiogenic isotopes, such as relaxation of stress regimes in upper plate domains. [1] DeCelles, P. G. et al. Nature Geoscience 2, 251-257 (2009) ; [2] Chapman, J. B. et al. Lithos 398- 299, (2021); [3] Lackey, J. S., et. al. J. Petrology 49, 1397–1426 (2008); [4] Turnbull, R. E. et al. Gondwana Res. 121, 436-471; [5] Ryan-Davis, J. et al. Contributions to Min. and Pet 174, 19 (2019)more » « less
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