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Tonalite, trondhjemite, and granodiorite (TTG) occur in dikes, plugs and tabular to equant plutons within the intrusive core of the Smartville Complex, a late Jurassic rifted arc. Two groups of TTG are recognized. A high-K group consisting of calc-alkaline tonalite to granodiorite is enriched in LILE and Th and depleted in Na, Y and HREE with respect to a more tholeiitic and trondhjemitic low-K group. Within the high-K group, Th, LIL, La, and La / Lu show a regional southward increase from biotite tonalite plutons in the north to granodiorite intrusions in the south. These regional chemical variations parallel regional chemical variations in older metavolcanic rocks and massive metadiabase that form the bulk of the basement into which the Smartville TTG were intruded. The geochemical and geological characteristics of most high-K group rocks are consistent with an origin by low-pressure (< 5 kb) partial melting of arc basement rocks. Some high-K group rocks, however, are strongly depleted in Y and HREE, suggestive of melting in the garnet stability field at P > 10 kb. Thus, the basement probed by the high-K group may be vertically, as well as laterally, extensive. A low-K group of largely tholeiitic tonalite to trondhjemite intrusions has lower LIL, Th, and La/Lu and higher Na, Y and HREE than the high-K group. Within this group, Y, Ga, and Na all increase to the west towards the Smartville sheeted dike complex. The westernmost intrusives in the low-K group have the chemical characteristics (e.g. high Y, Y/Nb and (Y+Nb)/Rb) of ocean ridge granites. The low-K group is best modeled by crystal fractionation of coeval, basaltic and andesitic magmas, although crustal assimilation also appears to be important in one of the intrusions.
Like most oceanic tonalites (e.g. plagiogranites), the low-K group rocks are overdepleted in LIL elements. The over-depletion appears to be an intrinsic property of the low-K intrusives, unrelated to post-magmatic hydrothermal effects. It is proposed that LIL elements are lost from low-K rocks because they evolve a vapor phase prior to the fixing of LIL elements by crystallization of a phase such as biotite. The relative order of LIL over-depletion (Rb > K > Ba) is consistent with this interpretation. Polygenetischer Tonalit-Tronhjemit-Granodiorit (TTG)-Magmatism.us im Smartville Komplex, Nord-Kalifornien, mit einer Notiz Über LILE Verarmung[PUBLICATION ABSTRACT]
Tonalite, trondhjemite, and granodiorite (TTG) occur in dikes, plugs and tabular to equant plutons within the intrusive core of the Smartville Complex, a late Jurassic rifted arc. Two groups of TTG are recognized. A high-K group consisting of calc-alkaline tonalite to granodiorite is enriched in LILE and Th and depleted in Na, Y and HREE with respect to a more tholeiitic and trondhjemitic low-K group. Within the high-K group, Th, LIL, La, and La / Lu show a regional southward increase from biotite tonalite plutons in the north to granodiorite intrusions in the south. These regional chemical variations parallel regional chemical variations in older metavolcanic rocks and massive metadiabase that form the bulk of the basement into which the Smartville TTG were intruded. The geochemical and geological characteristics of most high-K group rocks are consistent with an origin by low-pressure (< 5 kb) partial melting of arc basement rocks. Some high-K group rocks, however, are strongly depleted in Y and HREE, suggestive of melting in the garnet stability field at P > 10 kb. Thus, the basement probed by the high-K group may be vertically, as well as laterally, extensive. A low-K group of largely tholeiitic tonalite to trondhjemite intrusions has lower LIL, Th, and La/Lu and higher Na, Y and HREE than the high-K group. Within this group, Y, Ga, and Na all increase to the west towards the Smartville sheeted dike complex. The westernmost intrusives in the low-K group have the chemical characteristics (e.g. high Y, Y/Nb and (Y+Nb)/Rb) of ocean ridge granites. The low-K group is best modeled by crystal fractionation of coeval, basaltic and andesitic magmas, although crustal assimilation also appears to be important in one of the intrusions.
Like most oceanic tonalites (e.g. plagiogranites), the low-K group rocks are overdepleted in LIL elements. The over-depletion appears to be an intrinsic property of the low-K intrusives, unrelated to post-magmatic hydrothermal effects. It is proposed that LIL elements are lost from low-K rocks because they evolve a vapor phase prior to the fixing of LIL elements by crystallization of a phase such as biotite. The relative order of LIL over-depletion (Rb > K > Ba) is consistent with this interpretation. Polygenetischer Tonalit-Tronhjemit-Granodiorit (TTG)-Magmatism.us im Smartville Komplex, Nord-Kalifornien, mit einer Notiz Über LILE Verarmung[PUBLICATION ABSTRACT]
Springer-Verlag 1998