Petrogenetic and metallogenetic responses to Miocene slab flattening: new constraints from the El Indio-Pascua Au–Ag–Cu belt, Chile/Argentina

摘要

The Eocene–Miocene volcanic and hypabyssal rocks of the El Indio-Pascua Au–Ag–-Cu belt in the southern central Andean flat-slab region are medium–high-K calc-alkaline arc suites, ranging in composition from andesite to rhyolite. A significant transition in magmatic trace element chemistry, coinciding with a pronounced reduction in magma output, occurred in the late-Middle Miocene as documented by 40Ar–39Ar geochronology. The upper Eocene–lower-Middle Miocene rocks exhibit low Sr/Y ratios (<50), minor heavy REE fractionation with Sm/Yb ratios not exceeding 3.5 and, in some cases, minor negative Eu anomalies. In contrast, the largely dacitic rocks erupted after ca. 13 Ma are depleted in Y (≤10 ppm), have generally high, but variable Sr/Y ratios (30–200), exhibit moderate middle and heavy REE fractionation (Sm/Yb: 3.7–5.9) and lack negative Eu anomalies. The latter features are characteristic of adakitic suites (i.e. slab-melts), but the regional temporal and spatial distribution of arc magmatism precludes a major magma source in the downgoing slab. This evolution is interpreted as reflecting a progressive increase in pressure and the availability of water in the lower-crustal site of magma generation, establishing both garnet and hornblende as major stable phases in the residuum. The pressure in the lower crust increased in response to episodic crustal thickening related to the shallowing of the slab, a process recorded by the incision of three regional pediplains over the period 17–6 Ma. Elimination of the subarc asthenospheric mantle and much of the lithospheric mantle by ca. 10 Ma permitted direct incursion of slab-derived, highly oxidised metal- and volatile-rich supercritical fluids into the lower crust, stimulating melting of mafic, garnet amphibolitic and eclogitic assemblages.