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The trondhjemites of the Cachi Range (24°10′ to 25°07′S latitude and 66°10′ to 66°30′ W longitude) are a group of epizonal stocks that have a minimum Cambrian age. These postkinematic stocks intrude folded slates and graywackes of the crystalline basement of the Cachi Range in the Eastern Cordillera of northwestern Argentina. They are leucocratic rocks composed of plagioclase (An30 to An10) and quartz, with biotite, muscovite, and possible magmatic epidote as the principal accessory minerals. Minor amounts of microcline, cordierite, tourmaline, and garnet occur in some samples. A swarm of peraluminous pegmatites with rare-element mineralization formed by fractional crystallization of the main magma. The rocks are classified as high-alumina trondhjemites (average of 22 analyses: SiO2—71.82 percent; TiO2—0.20 percent; Al2O3—16.83 percent; Fe2O3t—1.53 percent; MnO—0.02 percent; MgO—0.52 percent; CaO—2.37 percent; Na2O—5.23 percent; K2O—0.85 percent; P2O5—0.06 percent; and Li—21 ppm; Rb—51 ppm; Sr—638 ppm; Y—19 ppm; Zr—150 ppm; Co—24 ppm; and Cr—17 ppm). These rocks are poor in LIL elements and have Rb/Sr ratios of 0.08, typical of continental trondhjemites. The Ba/CaO and Y/CaO ratios are consistent with an origin by partial melting of amphibolite. The data suggest that partial melting of a basaltic source is a more likely origin for the Cachi trondhjemites than a scheme involving fractional crystallization of a tholeiitic magma. We suggest that this magmatism must have been related to a continental-margin magmatic arc.

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