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Sisson, T.W., 1991

Field, geochemical, and experimental studies of aluminous arc magmas

Bibliographic Reference

Sisson, T.W., 1991, Field, geochemical, and experimental studies of aluminous arc magmas: Cambridge, Massachusetts, Massachusetts Institute of Technology, Ph.D. dissertation, 267 p., illust.


Field observations, geochemistry, and experimental petrology are used to interpret the origin, evolution, and volatile content of some high alumina basalts and basaltic andesites of modern and ancient magmatic arcs, and their role in the creation of more silicic magmas. Geochemistry of the mafic sill complex at Onion Valley, southeastern Sierra Nevada, California, and implications for the origin of Sierran granitoids. Sheeted mafic sills of the Jurassic intrusive complex at Onion Valley, southeastern Sierra Nevada, preserve evolved high alumina basalts through aluminous andesites that were present during growth of the Sierra Nevada batholith. Mafic magmas were water-rich. Many reached volatile saturation, as evidenced by miarolytic cavities. Onion Valley high alumina basalts differentiated to andesites by crystallization and separation of hornblende, calcic plagioclase, magnetite, and apatite. Associated sequences of layered cumulates were precipitated by magmas slightly more primitive than those preserved as sills. The magmas at Onion Valley probably originated as the derivative liquids of primitive wet high alumina basalts that differentiated at the base of the sub-Sierran crust, leaving ultramafic cumulates and evolved wet high alumina basalt liquids. Simple mixing calculations show that the major and trace element characteristics of average Sierran granodiorite are readily produced by a mixture of average basaltic sill from Onion Valley and average Sierran low-silica granite. This result supports models deriving Sierran granitoids from mixtures of crustal and mantle magmas. Characteristic chemical differences occur between typical mafic inclusions in Sierran granitoids and Onion Valley high alumina basalts. The differences are interpreted as resulting from chemical exchange between inclusions and host granitoid magmas. Experimental evidence for high water contents in some aluminous arc magmas. Phase relations of natural aphyric high alumina basalts and their intrusive equivalents were studied through rock-melting experiments at 2kb, water-saturated, with fO2 buffered at Ni-NiO. Experimental liquids saturated with olivine, calcic plagioclase, and either high calcium pyroxene or hornblende (+/- magnetite) have compositions close to those of many natural high alumina basalts and basaltic andesites with moderate-to-low MgO contents. Experimental solid phases, particularly plagioclase, match the compositions of phenocrysts observed in many natural high alumina basalts. The results lead to the interpretation that many mafic-to-intermediate aluminous arc magmas are very water-rich, commonly with H2O >4 wt. %, and have temperatures below 1,100oC. The wet magmas reach volatile-saturation in the upper crust. Continued ascent promotes decompression-degassing and the growth of plagioclase phenocrysts. Specific applications are made to the Aleutians, Fuego Volcano, Guatemala, and mafic sills of the Sierra Nevada batholith, California. The study supports models in which arc magmatism results from the introduction of water into the mantle wedge above the subducting slab. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)

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