Magma flow record of shallow plutons from AMS and petrographic fabric: Three cases from Central Chile
Résumé
Magma flow record on plutonic rocks is usually interpreted from mineral and magnetic fabric patterns. The shear caused by a crystallizing viscous flow can align minerals and leave a magnetic signature through the anisotropy of magnetic susceptibility (AMS), which should not be higher than 8%. In order to infer the magma flow patterns, in this contribution we present AMS and field-observed petrographic fabric data on three plutons with similar characteristics of composition and age, but its mineralogy heterogeneity increase southward. Our case studies are three well exposed shallow plutons that belongs to a NNW belt of 10 to 13 Ma quartz-monzonites in Central Chile, including, from North to South, La Gloria Pluton, San Gabriel pluton and Alfalfalito pluton.
In La Gloria Pluton the magnetic lineation is horizontal in NNW direction, following the main pluton elongation. The magnetic foliation changes regularly from vertical close to the walls, to horizontal at the center of the pluton. The petrographic fabric is generally weak and hard to recognize in the field, except in the central zones of the pluton, where it shows subvertical foliation and horizontal lineation. These data suggest whole scale convection and horizontal pluton propagation.
In San Gabriel Pluton the magnetic fabric also follows the borders of the pluton. The irregular shape of San Gabriel pluton give place to an irregular magma flow and consequently, the magnetic fabric. At the wall of the pluton, samples (1, 5, 3, 13, 12, 14, 17) have subvertical foliation following the borders of the pluton and NNW magnetic lineations; except at the southern and northern ends (2, 18, 11), where lineation and foliations are NEE. At the pluton interior (8) and under the roof (4), foliations are horizontal. The big wall exposed at the northern part of the pluton (7, 15, 16, 6) has a subvertical foliation in a NEE direction, suggesting that a subvertical magma flow and internal contacts inside the pluton (19, 10?). In general, these patterns are consistent with the observed petrographic fabric, which is well developed close to the borders, with two major trends between NNW and NEE (variation range of 60°), in detriment of the central middle levels of the pluton, where the fabric is isotropic.
The magnetic fabric of Alfalfalito Pluton also reflects the magma flow at walls and internal contacts. At the upper levels of the pluton (6, 3) magnetic lineation is subvertical, suggesting a vertical magma transport to upper levels. These three examples suggest that magma flow is conditioned by host rock at the limits of the magma reservoir, as well as, internal contacts, when magma with different mineral or enclave’s cargo coexists inside the reservoir.
This research has been supported by the National Fund for Scientific and Technological Development (FONDECYT) Project #1180577 granted by National Commission for Scientific and Technological Research (CONICYT) and the Fund for Publication Developments project [PEP I-2019071] granted by Universidad Mayor - Vicerrectoría de Investigación. I. Payacán was supported by the CONICYT doctoral grant [PFCHA-CONICYT #21151102].