Protracted late magmatic stage of the Caleu pluton (central Chile) as a consequence of heat redistribution by diking: Insights from zircon data and thermal modeling

Abstract

Zircon U-Pb geochronology and geochemistry are combined with whole-rock composition and thermal modeling to decipher the late magmatic stage of the composite Cretaceous Caleu pluton, which consists of four lithological zones: Gabbro-Diorite Zone (GDZ), Quartz Monzodiorite Zone (QMDZ), Granodiorite Zone (GZ) and Monzogranite Zone (MGZ). The four lithological zones include felsic dikes and veins of variable thickness and distribution. Zircons of four representative samples, each from the mentioned zones, were dated and chemically analyzed. The U-Pb ages exhibit sample-scale scatter derived from protracted zircon crystallization. At pluton scale the ages are substantially overlapped with a subtle decrease of ages from mafic to felsic sample; the latter has a normal age span distribution with a mean age of 94.68 +/- 0.71 (2 sigma confidence) and a MSWD of 0.95. Zircon grains from the uppermost zone of the pluton, where the QMDZ is emplaced, have the highest REE and HFSE contents. Zircon crystallization temperatures oscillate between 680 and 850 degrees C, regardless of the zircon age and sample composition. Differences in temperature and age of zircon crystallization of up to 185 degrees C and 2.6 Myr were identified at sample scale, respectively. Numerical modeling indicates that the melts from which zircon crystallized are highly crystalline (mostly higher than 60% crystal) and resemble MGZ in compositions. Time-dependent thermal models were performed to account for preservation of the system above solidus temperature for long time intervals consistent with those of zircon crystallization. Two non-exclusive scenarios for the late-stage development of Caleu pluton were considered: (i) pluton construction by magma pulses assembled incrementally and (ii) upward transport of residual melts by diking through a mush system to yield heat redistribution to the levels where the samples collected. The first scenario does not preserve residual melts for intervals as long as 2.6 Myr unless an extremely thick magma reservoir is considered. The second scenario is more favorable because it could provide enough heat that allows preserving residual melts above the solidus temperature depending on: (i) dike width, (ii) melt transport velocity and (iii) dike intensity (vol.% dike). For a dike width of 0.2 m and dike intensity of 10%, consistent with field observations, a transport velocity of 300 m/yr is required to maintain the upper mush zone above 700 degrees C. The melt transport would have occurred as successive events to allow developing the protracted late magmatic stage of the Caleu pluton.