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Geochemical Characteristics of Crustal Anatexis During the Formation of Migmatite at the Southern Sierra Nevada, California

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Geochemical Characteristics of Crustal Anatexis During the Formation of Migmatite at the Southern Sierra Nevada, California Contrib Mineral Petrol (2005) DOI 10.1007/s00410-005-0010-2 ORIGINAL PAPER Lingsen Zeng Æ Jason B. Saleeby Æ Mihai Ducea Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California Received: 6 October 2003 / Accepted: 10 March 2005 Ó Springer-Verlag 2005 Abstract We provide data on the geochemical and iso- tively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios topic consequences of nonmodal partial melting of a (0.15<Rb/Sr<1.0), and initial eNd values . Group B thick Jurassic pelite unit at mid-crustal levels that pro- leucosomes have relatively low Rb, Pb, Ba, and K2O duced a migmatite complex in conjunction with the contents, Rb/Sr ratios (<0.15), and initial eNd values. intrusion of part of the southern Sierra Nevada batho- The low Rb concentrations and Rb/Sr ratios of the lith at ca. 100 Ma. Field relations suggest that this pelitic group B leucosomes together suggest that partial melting migmatite formed and then abruptly solidified prior to was dominated by water-saturated or H2O-fluxed melt- substantial mobilization and escape of its melt products. ing of quartz + feldspar assemblage with minor Hence, this area yields insights into potential mid-crustal involvement of muscovite. Breakdown of quartz and level contributions of crustal components into Cordil- plagioclase with minor contributions from muscovite leran-type batholiths. Major and trace-element analyses resulted in low Rb/Sr ratios characterizing both group A in addition to field and petrographic data demonstrate and group B leucosomes. In contrast, group A leuco- that leucosomes are products of partial melting of the somes have greater contributions from K-feldspar, pelitic protolith host. Compared with the metapelites, which is suggested by: (1) their relatively high K con- leucosomes have higher Sr and lower Sm concentrations centrations, (2) positive or slightly negative Eu anoma- and lower Rb/Sr ratios. The initial 87Sr/86Sr ratios of lies, and (3) correlation of their Pb and Ba leucosomes range from 0.7124 to 0.7247, similar to those concentrations with K2O contents. It is also shown that of the metapelite protoliths (0.7125–0.7221). However, accessory minerals have played a critical role in regu- the leucosomes have a much wider range of initial eNd lating the partitioning of key trace elements such as Sm, values, which range from À6.0 to À11.0, as compared to Nd, Nb, and V between melt products and residues À8.7 to À11.3 for the metapelites. Sr and Nd isotopic during migmatization. The various degrees of parent/ compositions of the leucosomes, migmatites, and daughter fractionations in the Rb–Sr and Sm–Nd iso- metapelites suggest disequilibrium partial melting of the topic systems as a consequence of nonmodal crustal metapelite protolith. Based on their Sr, Nd, and other anatexis would render melt products with distinct iso- trace-element characteristics, two groups of leucosomes topic signatures, which could profoundly influence the have been identified. Group A leucosomes have rela- products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal dif- ferentiation, but also a potentially important process in Communicated by Ian Carmichael generating crustal-scale as well as individual pluton- scale isotopic heterogeneities. L. Zeng (&) Æ J. B. Saleeby Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA E-mail: [email protected] Tel.: +86-10-68992875 Fax: +86-10-68994781 Introduction M. Ducea The origin of geochemical and isotopic heterogeneities Department of Geological Sciences, University of Arizona, Tucson, AZ 85721, USA of large granitic batholiths is a fundamental question with regard to granitoid petrogenesis and the evolution L. Zeng Key Laboratory for Continental Dynamics, MLR, of the continental crust (Fyfe 1973; Miller et al. 1988; Institute of Geology, Chinese Academy of Geological Sciences, Sawyer 1998). Heterogeneity in the initial radiogenic Beijing 100037, China isotopic compositions of granitic plutons is commonly attributed to heterogeneities inherited from source re- ponents into Cordilleran-type batholiths. In this paper, gimes and preserved due to incomplete mixing of the we intend to address (1) the geochemical nature of the derivative magma. However, contamination of granitic melting products from the metapelite, (2) the magnitude magmas by assimilation of anatectic melts of wallrocks of isotopic heterogeneity produced by such crustal during ascent could also profoundly affect the geo- anatexis, (3) elemental and isotopic fractionations asso- chemical and isotopic character of the final products. ciated with the formation of the migmatites, and (4) The role of crustal anatexis at various crustal levels in implications for the intracrustal differentiation and iso- generating geochemical and isotopic heterogeneities is topic heterogeneities within Cordilleran-type batholiths. poorly understood. Thus, characterization of the geo- The nomenclature used in this paper follows that of chemical and isotopic nature of anatectic melts in high- Johannes (1988). Leucosome is the leucogranitic portion grade metamorphic terrains would help to better formed during migmatization, and the melanosome is the understand the sources of such heterogeneity and their mafic-rich selvage between the leucosome and meso- potential magnitude. some. Mesosome has intermediate color between the Recent experimental and field observations have leucosome and melanosome in a migmatite and differ- shown that partial melting at crustal levels is commonly entiates into leucosome and melanosome during mig- nonmodal (Vielzeuf and Holloway 1988; Le Breton and matization. The rock undergoing migmatization is called Thompson 1988; Rushmer 1991; Puziewicz and Johan- the parent rock, which in the nonmigmatitic far field is nes 1990; Inger and Harris 1993; Dardien et al. 1995; also referred to as the protolith. Harris et al. 1995) and produces melts with isotopic compositions that are in disequilibrium with respect to their sources (Tomomasini and Davies 1997; Knesel and Davidson 2002). Disequilibrium melting refers to any Geologic setting melting event where the liquid phase was not in chemical or isotopic equilibrium with the residue prior to melt Metamorphic rocks of the Isabella pendant consist of a extraction. Nonmodal melting is the melting process stratified sequence of pelitic and psammitic schist and where the proportion of reactant phases in the source is gneiss, quartzite, marble, and calc–silicate rock. This different from their proportions entering the melt. sequence of stratified rocks constitutes one of the best Crustal anatexis in response to thermal or chemical preserved sections through the early Mesozoic Kings perturbations is complex, depending on the temperature sequence, which constitutes a distinct family of pendants and pressure conditions, mineral compositions, and that runs for over 250 km along the axis of the southern water contents. Following a given temperature trajec- Sierra Nevada batholith (Saleeby and Bushy 1993). The tory and for mica-bearing metasedimentary rocks, entire pendant reached amphibolite facies conditions melting of quartz and feldspar assemblages is favored by during emplacement of and engulfment by the late the presence of excessive water and starts at a signifi- Cretaceous plutons. U/Pb zircon ages indicate the cantly lower temperature. In contrast, muscovite or emplacement of these plutons in the 100±2 Ma time biotite dehydration melting reactions occur at a much interval (Saleeby and Zeng 2005). higher temperature (e.g., Patin˜ o-Douce and Harris A geologic map of the area that was studied in detail 1998). In contrast to muscovite dehydration melting, within the Isabella pendant is shown in Fig. 1. The biotite dehydration melting is more complex because it is structure is characterized by a steeply SE-plunging iso- continuous over a wide range of temperatures. The clinal anticline with pelite units situated in core and temperature at which this reaction occurs depends on envelope positions of the fold. Metamorphic fabrics the composition of biotite (Mg/Fe ratio, Ti and F con- associated with the fold are L–S with the linear element centration) and the bulk Al content of source rocks (Le parallel to steeply plunging fold axes. Finite strain Breton and Thompson 1988; Vielzeuf and Montel 1994; studies on deformed pebble conglomerate in the Patin˜ o-Douce and Beard 1996; Stevens et al. 1997; quartzite unit indicate a stretch factor up to 1,000% Nabelek and Bartlett 2000). along the steep lineations. Homoaxial deformation in In this paper, we report the results of a detailed field satellite dikes from the Goat Ranch intrusion and leu- and geochemical study of a well-developed migmatite cosomes of the migmatitic pelite, as well as peak meta- zone in the Isabella pendant of the southern Sierra Ne- morphic textures, indicate that the isoclinal folding and vada batholith in California. This migmatite zone is of L–S fabric development occurred during pluton particular interest because it represents a single prograde emplacement. metamorphic sequence from greenshist facies metasedi- The upper pelite unit (J-pelite), which envelopes the ments to migmatites that lie in direct contact with the ca. large fold and is in contact with the Goat Ranch intru- 100 Ma Goat Ranch granodiorite pluton of the com- sion, has an apparent stratigraphic thickness in excess of posite batholith. Field relations
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