Ronda Peridotite

Overiew:

The Ronda peridotite massif represents a slice of continental lithospheric mantle which was exhumed during Miocene extension and emplaced into continental crust. The overlying crust displays extreme thinning and the underlying units show heating at shallow crustal levels, indicating a somewhat complex tectonic history (i.e. more than one faulting event). A better understanding of the regional geology requires research in multiple areas. We are conducting a study of the peridotite massif including the topics below.

This research is funded by NSF as part of an interdisciplanary effort to constrain the recent tectonics of the Alboran Sea region. PICASSO (Program to Investigate Convective Alboran Sea System Overturn) includes geophysicists working on tomography, magnetotellurics, and modeling, as well as geological research on the Ronda Peridotite and on young volcanics to detemine the initial and present states of the lithosphereic mantle and lower crust.

People involved in PICASSO at USC include John Platt (PI, Geology), Thorsten Becker (PI, Geophysics), Lisa Alpert (Ph.D. Student, Geophysics), Katy Johanesen (Ph.D. Student, Geology), Whitney Behr (Ph.D. Student, Geology), Chris Hames (Ph.D. Student, Geology), and Marlo Gawey (Undergraduate, Geology).

Petrology:

The Ronda peridotite body contains four distinct petrologically and structurally defined zones which are, from top to bottom:
1. Garnet-bearing peridotite mylonite along the northwestern boundary with overlying host rock. The peridotite contains garnet-pyroxenite layers which are in some places so sheared and boudinaged that only small "islands" of garnet surrounded by pyroxene remain.
2. Garnet- and spinel-bearing foliated peridotites containing pyroxenite layers.
3. Granular spinel-bearing peridotites containing pyroxenite and dunite layers.
4. Spinel- and plagioclase-bearing peridotites which are in some places granular and in others foliated.

This sequence, with high pressure garnet-peridotite at the top and lowest-pressure plagioclase-peridotite at the structural bottom, most likely indicates recrystallization of bottom-heated units during depressurization. That is, the entire body was once in the garnet field and only the top remained "chilled" enough to lock in that assemblage while the remaining rocks re-equilibrated in the spinel field, and the bottom-most rocks were still re-equilibrating in the plagioclase field.

Field-based and thin section petrography are both extremely important for working out the recrystallization and melt history of the peridotites. I have begun a detailed sample spread across the massif to detect subtle changes in grain size, chemistry, and mineralogy in the peridotites as well as the pyroxenite and dunite layers.

Geothermobarometry:

The pressure-temperature-time path of the peridotite body is important for constraining the the exhumation rates and timing of deformation in the region. I will measure P-T of both peridotite and pyroxenite layers along a detailed transect through each of the facies exposed in the Ronda peridotite massif. Additionally, I will use detailed thermometry to track abrubt changes within the massif which could indicate the presence of late ductile faults excising parts of the section.

Structure and Microstructures:

It is important to understand the relationship between foliation and rock type, as the loss of foliation is one of the main signals in the field to recognize the recrystallized zone. Detailed study of mineral relationships in all units, and especially across boundaries, will help give context to the thermobarometric results, tying them into regional structures to determine the timing of thermal events and depressurization.

Crustal Envelope:

The cover sequences above the peridotite body are best exposed near the Carratraca peridotite body. Here the metamorphic grade of crustal units decreases from kinzigites at the contact with the peridotite up through greenschist facies to phyllites and unmetamorphosed sediments over a scale of approximately 5 km (Argles et al., 1999).

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