Granite Petrogenesis in the Cordillera Real, Bolivia and Crustal Evolution in the Central Andes

Miller, James Fisher (1988). Granite Petrogenesis in the Cordillera Real, Bolivia and Crustal Evolution in the Central Andes. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.00010145

Abstract

Isotopic, trace and major element studies of intrusives from the Cordillera Real, Bolivia have identified two distinct granitoid types. Crustally derived granitoids are characterized by high Rb/Sr and Rb/Ba ratios, high degrees of peraluminosity, high eSr and Nd model ages of approximately 1 G a. Granitoids containing a mantle component are characterized by low Rb/Sr and Rb/Ba ratios, peraluminous or metaluminous character, low eSr and younger Nd model ages between 0.5 to 1 Ga.

Sr isotope geochronology has identified two periods of intrusion; late Carboniferous to early Permian and mid Miocene, both times of crustal thickening. Within plate and subduction related mantle components have been identified in specific granitoids on trace element grounds. Both AFC and simple mixing have been identified and quantitatively modelled as processes of combining mantle and crustal components to produce the least evolved samples studied.

The observed evolution of the granitoids was an isotopically closed system process and is modelled as fractional crystallization. The dominant role of accessory phases in controlling the REEs has been quantitatively modelled.

Intra-pluton variations in ESr and ENd can be modelled as either mixing between mantle and crust or episodic crustal remobilization. Individual plutons display homogeneous ESr and heterogeneous ENd values, reflecting the role of xenocrystic accessory phases in preserving the Nd isotope heterogeneity of the source.

The boundary between Hercynian and Andean Orogenies (250 Ma), is marked by: a sharp fall in Nd model ages of magmatic samples, (1 to 0.5 Ga); a fall in the calculated average source Rb/Sr ratio (0.6 to 0.2); and decrease in the inferred crustal reworking rate from between 1.75x Ga-i and 1.2x Ga-i to below 1.2x Ga b All these features reflect an increase in mantle involvement in the Andean orogeny relative to the Hercynian. A second shift in Nd model ages occurs at 25 Ma, the Nd model ages increase back to values comparable to the Hercynian data. Crustal thickening occurred at both these times, the first event probably involved magmatic underplating, the second was a purely intra-crustal event.

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