Adakitic magmatism and melt inclusions: investigating petrogenesis in the Cordillera Blanca, Peru.
| Executive Summary | |
|---|---|
| Researchers | Beverley Coldwell, CEESR Prof. J. Clemens, CEESR Dr M. Smith, CEESR Dr P. Murphy, CEESR |
| Funding Body/Source | Kingston University |
| Duration | 2005-ongoing |
| Project Summary | This project focuses on understanding magmatic systems and their petrogenetic associations, with particular reference to chemically rare rock types formed at both modern and Archaean collisional margins. In my present research, two separate Peruvian large volume ‘adakitic’ ignimbrites (Figure below) and associated granitoids are used to investigate magma generation at a tectonically complex margin. |
These rocks can also be used as proxies for <3 Ga Archaean tonalite-trondhjemite-granodiorite (TTG) suites, due to their chemical similarity. A number of tectonic factors may have influenced the magmatic source region in the Peruvian Cordillera, including over-thickened crust, the presence of a flat, cool, subducting slab, a cooling/frozen mantle wedge beneath the study region and rapid orogenic uplift. Petrological studies utilising geochemistry, melt-inclusion studies, experimental petrology and melt modelling have been used to determine a tectonic scenario most likely to represent that at 9°S in Peru. A simple garnet bearing mafic assemblage could generate the adakitic/TTG magma compositions, with the melt source region at ~2 GPa, and ~1100 °C. The melt shows no Pacific MORB associations, together with no significant crustal assimilation or alteration. Horizontally subducting MORB crust does not directly play a role in the ignimbrite melt generation, demonstrating that the popular notion of adakites being related to slab melting is questionable. While the Peruvian tectonic setting and environment should be perfect for adakite generation via slab melting according to many authors, my research contradicts this and contributes to growing evidence for adakite generation in other ways. This research proposes that batches of 15 to 25% partial melts of underplated, deep, mafic lower crust were channelled via a deep-seated fault, and either emplaced or erupted to form the Peruvian granitoids and ignimbrites. Melt inclusions are currently being analysed by laser ablation ICP-MS, in order to further constrain initial melt characteristics.
The identification of adakitic/TTG rocks that are not products of slab melt has implications for Archaean tectonics; particularly that subduction is not a requirement of TTG magma formation. Modern adakitic and Archaean TTG rock suites therefore have the potential to reveal a great deal about interactions between the mantle and melts, and melts and the lower/upper crust. Metasomatism, underplating, and mechanisms of magma generation are all potentially important in the generation of chemically unusual rocks. Using a combination of detailed field studies, experimental petrology, geochemical investigations, and melt inclusion studies, the history of chemically rare rock suites can be probed, in order to produce improved models of tectonic activity both at the present and > 3 Ga.
