Testing formation of the Torres del Paine Intrusive Complex, Southern Chile via in situ strontium isotope analyses: Insights into granite formation

Ortega, Kathlyn Rose

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https://hdl.handle.net/2142/121532 Copy

Description

Title

Testing formation of the Torres del Paine Intrusive Complex, Southern Chile via in situ strontium isotope analyses: Insights into granite formation

Author(s)

Ortega, Kathlyn Rose

Issue Date

2023-07-21

Director of Research (if dissertation) or Advisor (if thesis)

Lundstrom, Carig C

Committee Member(s)

Johnson, Thomas M

Department of Study

Earth Sci & Environmental Chng

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

M.S.

Degree Level

Thesis

Keyword(s)

• strontium isotopes
• geochemistry
• granite

Abstract

Granite formation processes have seen renewed debate, as our most-commonly-taught explanations for magma differentiation (fractional crystallization and partial melting) fail to explain geochronology and thermobarometry of many intrusive bodies. Here we assess whether the granite of the Torres del Paine Intrusive Complex (TDPIC) could have formed via reactions between low temperature (500°C) melt (LTM) and gabbro. The beautifully exposed contact between the TDPIC gabbro and overlying granite is sharp at the macro scale; however, mineral compositions smoothly transition between the two bodies, age dating contradicts cross cutting relations that suggest intrusion of granite into gabbro, and element analyses show that the mafic and felsic minerals are consistent with reaction toward chemical equilibrium. We hypothesize that the overlying granite formed through a process of melt-rock reaction, with pre-existing gabbro within the mafic suite transformed via heat input by underplating sills and migration of buoyant, alkalic, LTM upward. Previous whole-rock studies show 87Sr/86Sr of the gabbro consistent with those expected for a mantle magma, higher 87Sr/86Sr in the granite due to crustal input, and homogeneous lead isotope ratios between the gabbro and granite. In this study, however, we examine 87Sr/86Sr of spatially resolved spots within individual plagioclase phenocrysts. We find an overall pattern of lower 87Sr/86Sr in plagioclase as compared to that of whole rock values, with values varying by up to 0.0017. SEM EDS maps also show variation in plagioclase composition, with many plagioclase phenocrysts having more anorthitic cores relative to the rims. While more work is required to conclude melt-rock reaction as the mechanism of granite formation in the TDPIC, the data are consistent with expectations for granite forming from a pre-existing gabbro via interaction with a more felsic LTM. If it is true that such a process occurs at temperatures below those that we typically considered to be igneous, we must not only reassess current ideas about granite formation and the distinction between igneous and metamorphic processes, but pedagogical approaches for substantial parts of igneous petrology as a whole.

Graduation Semester

2023-08

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Kathlyn Ortega

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