Calc-alkaline rear-arc magmatism in the Fuegian Andes: Implications for the mid-cretaceous tectonomagmatic evolution of southernmost South America |
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| Authors: | Mauricio González Guillot Mónica Escayola Rogelio Acevedo |
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| Institution: | 1. Group of Dynamics of the Lithosphere (GDL), Institute of Earth Sciences Jaume Almera (ICTJA-CSIC), Lluís Solé i Sabarís s/n, 08028 Barcelona, Spain;2. Departamento de Geolog?a, Universidad de Oviedo, Oviedo, Spain;3. GEMOC-CCFS, Department of Earth and Planetary Sciences, Macquarie University North Ryde NSW, 2109, Australia;1. State Key Laboratory of Continental Dynamics, Northwest University, Xi''an, China;2. Department of Earth Sciences, Sun Yat-Sen University, Guangzhou, China;3. College of Earth Sciences, Guilin University of Technology, Guilin 541004, China;4. Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, 100101, China;5. State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;6. CSIRO Earth Sciences and Resource Engineering, Bentley, WA 6102, Australia |
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| Abstract: | The magmatic arc of the Fuegian Andes is composed mostly of Upper Mesozoic to Cenozoic calc-alkaline plutons and subordinated lavas. To the rear arc, however, isolated mid-Cretaceous monzonitic plutons and small calc-alkaline dykes and sills crop out. This calc-alkaline unit (the Ushuaia Peninsula Andesites, UPA) includes hornblende-rich, porphyritic quartz meladiorites, granodiorites, andesites, dacites and lamprophyres. Radiometric dating and cross-cutting relationships indicate that UPA is younger than the monzonitic suite. The geochemistry of UPA is medium to high K, with high LILE (Ba 500–2000 ppm, Sr 800–1400 ppm), HFSE (Th 7–23 ppm, Nb 7–13 ppm, Ta 0.5–1.1 ppm) and LREE (La 16–51 ppm) contents, along with relatively low HREE (Yb 1.7–1.3 ppm) and Y (9–19 ppm). The similar mineralogy and geochemistry of all UPA rocks suggest they evolved from a common parental magma, by low pressure crystal fractionation, without significant crustal assimilation. A pure Rayleigh fractionation model indicates that 60–65% of crystal fractionation of 60% hornblende + 34% plagioclase + 4% clinopyroxene + 1% Fe-Ti oxide, apatite and sphene (a paragenesis similar of UPA mafic rocks) can explain evolution from lamprophyres to dacites. The UPA has higher LILE, HFSE and LREE, and lower HREE and Y than the calc-alkaline plutons and lavas of the volcanic front. The HREE and Y are lower than in the potassic plutons as well. High concentrations of Th, Nb, Ta, Zr, Hf, LREE and Ce/Pb, and low U/Th, Ba/Th ratios in UPA, even in the least differentiated samples, suggest contributions from subducted sediments to the mantle source. On the other hand, relatively low HREE and Y, high LREE/HREE (La/Yb 11–38) ratios and Nb-Ta contents can be interpreted as mantle metasomatism by partial melts of either subducted garnetiferous oceanic sediment or basalt as well. Additionally, high LILE content in UPA, similar to the potassic plutons, suggests also a mantle wedge previously metasomatized by potassic parental magmas in their route to crustal levels. Therefore, UPA represents a unique suite in the Fuegian arc generated in a multiple hybridized source. UPA generation is related to a transition from normal to flat subduction which additionally caused the widening and landward migration of the magmatic arc, as well as crustal deformation. Rear-arc magmatism endured ca. 22 m.y.; afterwards, calc-alkaline magmatism remained at the volcanic front. |
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