CONNECTIONS BETWEEN SILICIC PLUTONISM AND CALDERA SYSTEMS: A NON-TRADITIONAL STABLE ISOTOPE STUDY OF THE CORDILLERA DEL PAINE

The relationship between silicic plutons and silicic caldera systems has puzzled Earth scientists for decades. Given recent shifts to views of plutons forming by slow incremental emplacement, the puzzle has increased. Here we present new Fe, U and Si isotope data for the Cordillera del Paine (CP) igneous complex in southern Chile. The CP consists of a 1 km vertical exposure of relatively homogenous granite overlying a nearly contemporaneous and possibly cogenetic 0.5 km mafic suite containing a zone of diorites underlain by hornblende gabbros. High precision geochronology indicates incremental emplacement [1].

Samples previously characterized by Michael [2,3] were analyzed by MCICPMS. δ⁵⁶Fe hyperbolically increases with increasing SiO₂ reaching values as high as 0.80 in aplites. Preliminary data indicate δ³⁰Si and δ²³⁸U also increase with differentiation. The cause of these variations remains debated between: 1) fractional crystallization; 2) late stage fluid removal [4]; 3) thermal gradient based differentiation[5]; and 4) crustal contamination. Radiogenic isotopes show a strong correlation between ⁸⁷Sr/⁸⁶Sr and SiO₂ but remarkably constant Pb with ²⁰⁸Pb/²⁰⁴Pb ~38.72, ²⁰⁷Pb/²⁰⁴Pb ~15.64 and ²⁰⁶Pb/²⁰⁴Pb ~18.83.

Comparison of the CP differentiation suite to silicic ignimbrite suites show both have the same trends of non-traditional stable isotope ratios with differentiation. This would tend to argue against silicic ignimbrites being melts extracted from shallow pluton mushes as they are not complementary; however this conclusion depends on the cause of the isotope fractionation which we continue to assess. Understanding the origin of non-traditional stable isotopes in differentiated silicic plutons and corresponding silicic extrusive rocks will provide important constraints to clarifying their enigmatic relationship.

[1] Michel et al. (2008) Geology 36, 459-462. [2] Michael (1984) Cont. Min, Petrol.87, 179-195. [3] Michael (1991) Cont. Min. Petrol. 108, 396-418. [4] Heimann A. et al. (2008) GCA 72, 4379-4396. [5] Lundstrom (2009) GCA 73, 5709-5729.