Early Cretaceous high-Ti and low-Ti mafic magmatism in Southeastern Tibet: Insights into magmatic evolution of the Comei Large Igneous Province

The Dala diabase intrusion, at the southeastern margin of the Yardoi gneiss dome, is located within the outcrop area of the ~ 132 Ma Comei Large Igneous Province (LIP), the result of initial activity of the Kerguelen plume. We present new zircon U-Pb geochronology results to show that the Dala diabase was emplaced at ~ 132 Ma and geochemical data (whole-rock element and Sr-Nd isotope ratios, zircon Hf isotopes and Fe-Ti oxide mineral chemistry) to confirm that the Dala diabase intrusion is part of the Comei LIP. The Dala diabase can be divided into a high-Mg/low-Ti series and a low-Mg/high-Ti series. The high-Mg/low-Ti series represents more primitive mafic magma compositions that we demonstrate are parental to the low-Mg/high-Ti series. Fractionation of olivine and clinopyroxene, followed by plagioclase within the low-Mg series, lead to systematic changes in concentrations of mantle compatible elements (Cr, Co, Ni, and V), REEs, HFSEs, and major elements such as Ti and P. Some Dala samples from the low-Mg/high-Ti series contain large ilmenite clusters and show extreme enrichment of Ti with elevated Ti/Y ratios, likely due to settling and accumulation of ilmenite during the magma chamber evolution. However, most samples from throughout the Comei LIP follow the Ti-evolution trend of the typical liquid line of descent (LLD) of primary OIB compositions, showing strong evidence of control of Ti contents by differentiation processes. In many other localities, however, primitive magmas are absent and observed Ti contents of evolved magmas cannot be quantitatively related to source processes. Careful examination of the petrogenetic relationship between co-existing low-Ti and high-Ti mafic rocks is essential to using observed rock chemistry to infer source composition, location, and degree of melting

Geologic map of the Goat Ranch area, Southern Sierra Nevada, California: Supplement 1 from "Non-modal partial melting of metasedimentary pendants in the southern Sierra Nevada and implications for the deep origin of within-pluton isotopic heterogeneity" (Thesis)

Results from field mapping, structural analysis, major and trace element geochemistry, and radiogenic isotopic data from the Goat Ranch migmatite complex on the south shore of Lake Isabella, southern Sierra Nevada, California, are presented to (1) determine the major and trace element, Sr and Nd isotopic compositions of anatectic melts from pelitic sources; (2) investigate the structural and metamorphic responses of the Isabella pendant to the emplacement of the Goat Ranch pluton at ~100 Ma; and (3) evaluate the magnitude of assimilation of metasedimentderived melts by the Goat Ranch pluton at a mid-crustal level. Nd and Sr isotopic compositions were measured on the non-migmatitic distal wallrocks, leucosomes, migmatites, traverses into the Goat Ranch pluton, leucogranite dikes, and samples from the Rabbit Island, Heal Peak, and Bob Rabbit plutons. Major and trace element analyses were performed on selected samples of the leucosomes, migmatites and metapelites. Major and trace element analyses in addition to field and petrographic data demonstrate that leucosomes are products from partial melting of the pelitic protolith host. These data show that (1) contamination of Goat Ranch intrusion is restricted to the immediate proximity to migmatitic wallrocks. Isotopic heterogeneity of the Goat Ranch intrusion is not related to assimilation at or near the level of exposure, but from a deeper source; (2) as compared to the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. Sr and Nd isotope compositions of leucosomes, migmatites and metapelites suggesting a disequilibrium partial melting of the metapelite protolith; (3) based on their Sr, Nd and other trace element characteristics, two groups of leucosomes have been identified. Group A leucosomes show distinct positive Eu anomalies, relatively high Rb, Pb, Ba and K2O contents, and low Rb/Sr ratios. Group B leucosomes have negative Eu anomalies, relatively low Rb, Pb, Ba and K2O contents, and low Rb/Sr ratios as well; (4) the leucogranite dikes also can be subdivided into Group A (high 87Sr/86Sr(T) and low eNd(T)), and Group B (low 87Sr/86Sr(T)) and high eNd(T)); (5) H2O-fluxed melting of quartz + plagioclase with minor involvement of muscovite melting dominated the leucosome production; (6) Group A leucogranite dikes resulted from partial melting of the lower pelite, and Group B dikes from partial melting of the upper pelite; and (7) the Bob Rabbit pluton represents an extreme end-member case that was derived completely from melting of the upper pelite or its equivalent in depth in the I-SCR (strongly contaminated and reduced I-type pluton) zone. Strain analysis shows that progressive partial melting resulted in the loading framework transition in the upper pelite unit from LBF structure (the stronger phase forms a load-bearing framework) to IWL structure (the weaker phase forms an interconnected weak matrix) with proximity to the pluton. The presence of melts has greatly affected the strain partitioning within the migmatite zone. By incorporating accessory phase dissolution kinetics into non-modal partial melting of metasedimentary sources, theoretical modeling shows that non-modal partial melting of a pelitic source results in melts following two paths in eNd-87Sr/86Sr ratio space. Path 1 represents those partial melting reactions that favor muscovite/biotite dehydration and apatite but not monazite dissolution, leading to melts with elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and eNd values. In contrast, Path 2 represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role, and favor monazite over apatite dissolution, and lead to melts with lower Rb/Sr, 87Sr/86Sr, Sm/Nd, and eNd values than their sources

Non-Modal Partial Melting of Metasedimentary Pendants in the Southern Sierra Nevada and Implications for the Deep Origin of Within-Pluton Isotopic Heterogeneity

Results from field mapping, structural analysis, major and trace element geochemistry, and radiogenic isotopic data from the Goat Ranch migmatite complex on the south shore of Lake Isabella, southern Sierra Nevada, California, are presented to (1) determine the major and trace element, Sr and Nd isotopic compositions of anatectic melts from pelitic sources; (2) investigate the structural and metamorphic responses of the Isabella pendant to the emplacement of the Goat Ranch pluton at ~100 Ma; and (3) evaluate the magnitude of assimilation of metasedimentderived melts by the Goat Ranch pluton at a mid-crustal level. Nd and Sr isotopic compositions were measured on the non-migmatitic distal wallrocks, leucosomes, migmatites, traverses into the Goat Ranch pluton, leucogranite dikes, and samples from the Rabbit Island, Heal Peak, and Bob Rabbit plutons. Major and trace element analyses were performed on selected samples of the leucosomes, migmatites and metapelites. Major and trace element analyses in addition to field and petrographic data demonstrate that leucosomes are products from partial melting of the pelitic protolith host. These data show that (1) contamination of Goat Ranch intrusion is restricted to the immediate proximity to migmatitic wallrocks. Isotopic heterogeneity of the Goat Ranch intrusion is not related to assimilation at or near the level of exposure, but from a deeper source; (2) as compared to the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. Sr and Nd isotope compositions of leucosomes, migmatites and metapelites suggesting a disequilibrium partial melting of the metapelite protolith; (3) based on their Sr, Nd and other trace element characteristics, two groups of leucosomes have been identified. Group A leucosomes show distinct positive Eu anomalies, relatively high Rb, Pb, Ba and K2O contents, and low Rb/Sr ratios. Group B leucosomes have negative Eu anomalies, relatively low Rb, Pb, Ba and K2O contents, and low Rb/Sr ratios as well; (4) the leucogranite dikes also can be subdivided into Group A (high 87Sr/86Sr(T) and low eNd(T)), and Group B (low 87Sr/86Sr(T)) and high eNd(T)); (5) H2O-fluxed melting of quartz + plagioclase with minor involvement of muscovite melting dominated the leucosome production; (6) Group A leucogranite dikes resulted from partial melting of the lower pelite, and Group B dikes from partial melting of the upper pelite; and (7) the Bob Rabbit pluton represents an extreme end-member case that was derived completely from melting of the upper pelite or its equivalent in depth in the I-SCR (strongly contaminated and reduced I-type pluton) zone. Strain analysis shows that progressive partial melting resulted in the loading framework transition in the upper pelite unit from LBF structure (the stronger phase forms a load-bearing framework) to IWL structure (the weaker phase forms an interconnected weak matrix) with proximity to the pluton. The presence of melts has greatly affected the strain partitioning within the migmatite zone. By incorporating accessory phase dissolution kinetics into non-modal partial melting of metasedimentary sources, theoretical modeling shows that non-modal partial melting of a pelitic source results in melts following two paths in eNd-87Sr/86Sr ratio space. Path 1 represents those partial melting reactions that favor muscovite/biotite dehydration and apatite but not monazite dissolution, leading to melts with elevated Rb/Sr, 87Sr/86Sr, Sm/Nd, and eNd values. In contrast, Path 2 represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role, and favor monazite over apatite dissolution, and lead to melts with lower Rb/Sr, 87Sr/86Sr, Sm/Nd, and eNd values than their sources.</p

Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California

We provide data on the geochemical and isotopic consequences of nonmodal partial melting of a thick Jurassic pelite unit at mid-crustal levels that produced a migmatite complex in conjunction with the intrusion of part of the southern Sierra Nevada batholith at ca. 100 Ma. Field relations suggest that this pelitic migmatite formed and then abruptly solidified prior to substantial mobilization and escape of its melt products. Hence, this area yields insights into potential mid-crustal level contributions of crustal components into Cordilleran-type batholiths. Major and trace-element analyses in addition to field and petrographic data demonstrate that leucosomes are products of partial melting of the pelitic protolith host. Compared with the metapelites, leucosomes have higher Sr and lower Sm concentrations and lower Rb/Sr ratios. The initial ^(87)Sr/^(86)Sr ratios of leucosomes range from 0.7124 to 0.7247, similar to those of the metapelite protoliths (0.7125–0.7221). However, the leucosomes have a much wider range of initial ε_(Nd) values, which range from −6.0 to −11.0, as compared to −8.7 to −11.3 for the metapelites. Sr and Nd isotopic compositions of the leucosomes, migmatites, and metapelites suggest disequilibrium partial melting of the metapelite protolith. Based on their Sr, Nd, and other trace-element characteristics, two groups of leucosomes have been identified. Group A leucosomes have relatively high Rb, Pb, Ba, and K_2O contents, Rb/Sr ratios (0.15<Rb/Sr<1.0), and initial ε_(Nd) values. Group B leucosomes have relatively low Rb, Pb, Ba, and K_2O contents, Rb/Sr ratios (<0.15), and initial ε_(Nd) values. The low Rb concentrations and Rb/Sr ratios of the group B leucosomes together suggest that partial melting was dominated by water-saturated or H_2O-fluxed melting of quartz + feldspar assemblage with minor involvement of muscovite. Breakdown of quartz and plagioclase with minor contributions from muscovite resulted in low Rb/Sr ratios characterizing both group A and group B leucosomes. In contrast, group A leucosomes have greater contributions from K-feldspar, which is suggested by: (1) their relatively high K concentrations, (2) positive or slightly negative Eu anomalies, and (3) correlation of their Pb and Ba concentrations with K_2O contents. It is also shown that accessory minerals have played a critical role in regulating the partitioning of key trace elements such as Sm, Nd, Nb, and V between melt products and residues during migmatization. The various degrees of parent/daughter fractionations in the Rb–Sr and Sm–Nd isotopic systems as a consequence of nonmodal crustal anatexis would render melt products with distinct isotopic signatures, which could profoundly influence the products of subsequent mixing events. This is not only important for geochemical patterns of intracrustal differentiation, but also a potentially important process in generating crustal-scale as well as individual pluton-scale isotopic heterogeneities

Nd isotope disequilibrium during crustal anatexis: A record from the Goat Ranch migmatite complex, southern Sierra Nevada batholith, California

Geological and geochemical studies of a pelitic migmatite complex within the Isabella pendant of the southern Sierra Nevada batholith, California, suggest that the leucosomes represent the products of partial melting of the metapelite host driven by the emplacement of the adjacent Goat Ranch pluton ca. 100 Ma. The leucosomes preserve a record of large-magnitude Nd isotope disequilibrium with respect to their pelitic source. The leucosomes have a wide range of ε_(Nd(100 Ma)) from −6.0 to −11.0, as compared to −8.7 to −11.3 for the source. They can be subdivided into two groups based on their major elements and Sr and Nd isotope geochemistry. Group I leucosomes have higher P_2O_5 contents and ε_(Nd(100 Ma)) values than those of group II. The ε_(Nd(100 Ma)) values of group I leucosomes are significantly higher than those of metapelites and migmatites by two to four epsilon units, suggesting that group I leucosomes are in Nd isotope disequilibrium with their sources. Correlations among P_2O_5 contents, ε_(Nd(100 Ma)) values, and Sm/Nd ratios in the leucosomes suggest that apatite or monazite has played a dominant role in fractionating Sm from Nd and generating Nd isotope disequilibrium. Dissolution of apatite or monazite might play a critical role in regulating the behavior of the Sm-Nd isotope systems and thus the Nd isotope compositions of melts generated during crustal anatexis, especially in metasedimentary protoliths

Contrasting geochemical signatures of fluid-absent versus fluid-fluxed melting of muscovite in metasedimentary sources: The Himalayan leucogranites

Most of the Himalayan Cenozoic leucogranites are products of partial melting of metapelite sources. In the Malashan-Gyirong area (southern Tibet), the geochemical compositions of leucogranites define two groups with distinct whole-rock major elements, large ion lithophile elements, rare earth elements, high field strength elements, and Sr and Hf isotope ratios. Based on published experimental results that define generalized melting reactions of metapelitic sources, we infer that these leucogranites are the products of two different types of crustal anatexis: fluid-fluxed melting and fluid-absent melting of muscovite in metasedimentary sources. As compared to the leucogranites derived from fluid-absent melting, those from fluid-fluxed melting have relatively higher Ca, Sr, Ba, Zr, Hf, Th, and light rare earth element concentrations, and Zr/Hf, Eu/Eu*, and Nd/Nd*, but lower Rb, Nb, Ta, and U concentrations, Rb/Sr and ^(87)Sr/^(86)Sr ratios, and ε_(Hf)(t). The geochemical differences can be explained by melting behaviors of major (muscovite, feldspar) and accessory minerals (zircon and monazite) during different modes of crustal anatexis. The systematic elemental and isotopic signatures of different types of crustal anatexis and, in particular, the coupling of major and trace elements that results from common influences on rock-forming and accessory mineral behaviors provide tools with which to refine our understanding of the nature of crustal anatexis

Limited migration of leucosomes in a migmatite and effects of progressive partial melting on strain partitioning

The migration of melts in the crust and mantle is one of important problems in geology. As a product of crustal anatexis, leucosomes in a migmatite provide a unique opportunity to investigate the factors that affects the migration and transportation of crustal melts in the lower and middle continental crust. We carried out a detailed major element geochemistry and structural analyzes on a suite of leucosomes from the Goat Ranch migmatite complex, Southern Sierra Nevada, California, and theoretical calculations to evaluate their migration distances. Field observations show that: (1) the leucosomes have a thickness ranging from millimeter to centimeter and (2) metamorphic temperatures, metamorphic grades, and degrees of partial melting in the Cretaceous metapelite increase with the decreasing proximity of the Goat Ranch pluton. In the localities of partial melting of high degrees (>10%), the loading framework is dominated by IWL (Interconnected Weak Layers) in which the strain is accommodated by weak interconnected leucosomes. In contrast, LBF (Load bearing framework) dominated those areas having a low degree of partial melting (<5%) where leucosomes occurred as isolated blobs or pockets. The strain was accommodated by strong matrix. It seems that melts represented by the leucosomes acts as weak phases during deformation. During migmatization, the presence of melts greatly affects the bulk strength and the strain partitioning for a rock undergoing syn-metamorphism deformation. Using the Shaw's viscosity model and modified Stoke's equation, we calculated the viscosities of the leucosomes based on their major element compositions and estimated their migration distance. These results show that these leucosomes had experienced limited migration due to high viscosities. They have viscosities ranging from 10^9 to 10^(12)Pas, which are 3 to 8 orders of magnitude higher than 10^4 ~ 10^6 Pas for typical granites. Results from this study provide strong support for using migmatites to characterize the geochemical and isotopic geochemistry of crustal anatexis and examine the mechanic properties of molten rocks

Coupling of anatectic reactions and dissolution of accessory phases and the Sr and Nd isotope systematics of anatectic melts from a metasedimentary source

Advances in field observations and experimental petrology on anatectic products have motivated us to investigate the geochemical consequences of accessory mineral dissolution and nonmodal partial melting processes. Incorporation of apatite and monazite dissolution into a muscovite dehydration melting model allows us to examine the coupling of the Rb-Sr and Sm-Nd isotope systems in anatectic melts from a muscovite-bearing metasedimentary source. Modeling results show that (1) the Sm/Nd ratios and Nd isotopic compositions of the melts depend on the amount of apatite and monazite dissolved into the melt, and (2) the relative proportion of micas (muscovite and biotite) and feldspars (plagioclase and K-feldspar) that enter the melt is a key parameter determining the Rb/Sr and ^(87)Sr/^(86)Sr ratios of the melt. Furthermore, these two factors are not, in practice, independent. In general, nonmodal partial melting of a pelitic source results in melts following one of two paths in ε_(Nd^-) ^(87)Sr/^(86)Sr ratio space. A higher temperature, fluid-absent path (Path 1) represents those partial melting reactions in which muscovite/biotite dehydration and apatite but not monazite dissolution play a significant role; the melt will have elevated Rb/Sr, ^(87)Sr/^(86)Sr, Sm/Nd, and ε_(Nd) values. In contrast, a lower temperature, fluid-fluxed path (Path 2) represents those partial melting reactions in which muscovite/biotite dehydration plays an insignificant role and apatite but not monazite stays in the residue; the melt will have lower Rb/Sr, ^(87)Sr/^(86)Sr, Sm/Nd, and ε_(Nd) values than its source. The master variables controlling both accessory phase dissolution (and hence the Sm-Nd system), and melting reaction (and hence the Rb-Sr systematics) are temperature and water content. The complexity in Sr-Nd isotope systematics in metasediment-derived melts, as suggested in this study, will help us to better understand the petrogenesis for those granitic plutons that have a significant crustal source component