Incremental growth of an upper crustal, A-type pluton, Argentina: evidence of a re-used magma pathway

ROCHER, SEBASTIÁN; MEMETI, VALBONE; MACCHIOLI GRANDE, MARCOS; LARROVERE, MARIANO A.; BASEI, MIGUEL A.S.; GALINDO, CARMEN; ALASINO, PABLO H.; DAHLQUIST, JUAN A.; PATERSON, SCOTT R.

San Miguel de Tucumán

Congreso; XX Congreso Geológico Argentino; 2017

Universidad Nacional de Tucumán

Carboniferous igneous activity in the Sierra de Velasco (NW Argentina) led to the emplacement of several magma bodies at shallow levels (<2 kbar). One of these, the San Blas intrusive complex formed over <2-3 m.y. during three periods of magma additions characterized by variations in magma sources and emplacement styles. The main units, mostly felsic granitoids, have U-Pb crystallization ages that overap within error. Using cross-cutting relationships, the main intrusive units fro older to younger are: (1) the Asha unit (340 ± 7 Ma): a tabular to funnel-shaped intrusion with contacts discordant to regional host rock structures, emplaced during a regional strain field dominated by ~WSW-ENE shortening; (2) the San Blas unit (344 ± 2 Ma): an approximately cylindrical-shaped intrusion formed by multiple batches of magmas, with a roughly concentric fabric pattern and displacement of the host rock by ductile flow resulting in 35% of shortening based on deflection of host rock markers in the wall (e.g. mylonitic foliation); and (3) the Hualco unit (346 ± 6 Ma): a small body with a possible mushroom geometry and contacts concordant to regional host rock structures. The magma pulses making up these units, define two groups of A-typw granitoids. The first group includes the peraluminous granitic rocks of the Asha unit generated mostly by crustal sources (εNdi=-5.8 and εHfi in zircon= -2.9 to -4.5). The second group comprises the metaluminous to peraluminous granitic rocks of the youngest units (San Blas and Hualco), which were formed by a heterogeneous mixture between mantle and crustal sources (εNdi=+0.6 to -4.8 and εHfi in zircon= +3.1 to -6). Sr and Pb isotope analyses conducted on the core and the rim of K-feldspaer megacryst collected in the San Blas unit display from core to rim more primitive isotope composition patterns. Our results provide a comprehensive view of the evolutionof an intrusive complex formed from multiple non-consanguineous magma intrusions that ascended up the same magmatic plumbing system during mostly downward transfer of host materials. As the plutonic system matured, several chambers grew at shallow crustal levels, leading to complex interactions between liquid-liquid and liquid-solid materials that are not in isotopic equilibrium. The ascent of magmas is governed by the visco-elastic flow of host rock that for younger batches include oldr hot magma mush. The latter results in downward flow of older, during rise of younger magma. Such complexes may reflect the plutonic feeders to volcanic centers where chemically distinct magmas are erupted from multiple ephemeral chambers.