科研之友
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摘要
Four Middle-to-Late Miocene barren plutonic complexes that occur between the giant porphyry copper deposits of the central Chilean Andes were selected for U-Pb LA-ICPMS geochronology and Hf-isotope systematics on single zircon grains. Major and trace elements and Sr-Nd-Hf isotope whole rock geochemical studies were under-taken to compare with slightly younger or coeval barren and fertile intrusive rocks between 32～ and 34～S. The studied granitoids yield resolvable crystallization ages of 11.3㊣0.1 Ma (Cerro Mes車n Alto massif), 10.3㊣0.2 Ma (La Gloria pluton), 14.9㊣0.2 Ma/14.9㊣0.1 Ma (Yerba Loca stock) and 11.2㊣0.1 Ma/14.7㊣0.1 Ma (San Francisco Batholith). Major and trace elements discard an adakitic signature as suggested for coeval porphyric intrusions at 32～S, slightly younger mineralized porphyries at R赤o Blanco-Los Bronces deposit and other Cenozoic adakites. Volcanic host rocks are less fractionated than the intrusive rock units. The same observation can be made for the unmineralized northern plutons compared to the southern ones. Initial Sr-Nd isotope data show insignificant variation (0.703761-0.704118 and 0.512758-0.512882), plotting in the mantle array. Trace element enrichment can be explained by addition of subducted-slab fluids and/or terrigenous sediments to the mantle wedge prior to and/or slight crustal input during magma ascent. Zircon grains separated from these barren intrusives share a similar initial 汍Hf i-data variation for the younger age group (10-12 Ma; 7.04-9.54) and show a more scattered range for the older one (14-15 Ma; 8.50-15.34); both sets plot between the DM and CLTUR evolution lines. There is evidence that magma evolution was slightly distinct through time from older to younger barren magmatism, compared to a few fertile porphyritic rocks from R赤o Blanco-Los Bronces porphyry copper deposit. It is suggested that chronological inconsistencies within these complexes might be related to differential shortening across the NE-SW-trending Yeso Valley transfer fault, assumed as coeval, which also explains the local lack of easterly magmatic are front migration. Cuatro complejos plut車nicos del Mioceno Temprano a Medio que se encuentran ubicados entre los dep車sitos de p車rfidos gigantes de cobre de los Andes chileno centrales fueron seleccionados para un estudio de geocronolog赤a U-Pb por LA-ICPMS y de is車topos de Hf en granos individuales de circ車n. Estudios geoqu赤micos de elementos mayores y trazas y los is車topos de Sr-Nd-Hf en roca permiten compararlos con rocas intrusivas ligeramente m芍s j車venes o coet芍neas est谷riles y f谷rtiles ent
