Emplacement of the Las Tazas plutonic complex, coastal cordillera, Northern Chile

Wilson, Jeff (1996) Emplacement of the Las Tazas plutonic complex, coastal cordillera, Northern Chile. (PhD thesis), Kingston University, uk.bl.ethos.319720.


The Las Tazas Complex consists of two plutons; a granodioritic northern pluton and a monzonitic southern pluton. These were both emplaced during the Early Cretaceous along the Atacama Fault Zone during a change from dip-slip to transcurrent displacement. The Las Tazas Complex is a syntectonic intrusion. This is hased primarily on; (a) a fundamental agreement between plutonic and regional kinematic frameworks; (b) the presence of a down-temperature fabric continuum along the eastern contact of the Complex and; (c) melt-filled C' - surfaces in the wall-rocks. The Complex was emplaced under local contractional conditions during partitioned transpression. The northern pluton was emplaced during partitioned oblique dextral transpression, while the southern pluton was emplaced during east-side-down compressive dip-slip displacement. The transpressive setting suggests that the magma has actively dilated the fault surfaces under contractional conditions, rather than simply "filling in" space in dilatational jogs. Lithological zoning within the Las Tazas Complex has resulted from the open-system multiple injection of discrete magma hatches, not from the closed-system in-situ fractionation of a single parental magma body. Emplacement was characterised by periods of the multiple injection of small magma batches during fairly continuous magma production, punctuated by periods of quiescent magma production, represented by internal contacts. Emplacement-level mixing and fractionation has only occurred on a local scale during the evolution of the plutons. Geochemical models of in-situ crystallisation produce ambiguous non-unique models which can only be constrained by clear field observation. Strain-states associated with emplacement are localised phenomena, induced by a local interaction between magma supply and wall-rock displacement. On this basis strain-states in the vicinity of plutons do not reflect regional strain-states. As such, the kinematic framework and strain state associated with the emplacement of the Las Tazas Complex cannot be used to constrain the regional strain-state during the construction of the Cretaceous magmatic arc.

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