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Title: The geology and geochronology of the Arequipa Segment of the Coastal Batholith of Peru
Author: Moore, Nigel Denis
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 1979
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Abstract:
This project examines in detail part of the Arequipa Segment of the Coastal Batholith of Peru. The batholith and its envelope were mapped in the environs of lca in southern Peru. The envelope ranges in age from Upper Jurassic to Middle Cretaceous and the stratigraphy relects an early phase of relatively slow subsidence and the accumulation of quartzites, volcanics and limestones. This was followed by a shorter phase, in the Albian, in which subsidence rates increased dramatically and the eruption and accumulation of a thick pile of submarine andesitic volcanics took place. This local stratigraphy has been correlated regionally and extends the known stratigraphical framework of the coastal region of Peru. The batholith is a multi-component body with a compositional range from gabbro to syenogranite. As in the more well known Lima,Segment of the Coastal Batholith it is possible to define units and super-units and to establish a relative chronology between them. Thus, in decreasing order 'of age, they are the Patap, Pampahuasi, lncahuasi, Linga and Tiabaya Super-units. The Patap super-unit is gabbroic in composition and forms an extension of the early gabbroic precursors to the batholith found in northern Peru. The pampahuasi Superunit consists mainly of tonalites which previously had been thought to be associated with either the Incahuasi or Tiabaya Super-units. The mapping in the Ica region and the subsequent petrographical examinations and modal analyses suggest that they are a super-unit in their own right. The Incahuasi, Linga and Tiabaya Super-units have been recognised elsewhere in the Arequipa Segment. They are all intermediate in bulk composition with rock types of quartzmonzodiorite and granodiorite predominating. This thesis presents 28 K-Ar ages on the major plutonic rock types in the region. This dating reveals that the batholith was emplaced in two stages. An initial episode of plutonism occurred between 104 - 97 Ma. within which the Patap, Pampahuasi, Incahuasi and Linga Super-units were emplaced. This was followed by a period of inactivity lasting 12 Ma. until the Tiabaya Super-unit was emplaced between 85 - 77 Ma.. The radiometric dating also shows that many of the K-Ar ages obtained from the older superunits are re-set ages which are similar to those obtained from the Tiabaya Super-unit. This strengthens a theory formed in the field that many of the outcrops of the older rocks, especially the Incahuasi outcrops in the east of the region, are underlain at shallow depths hy a large pluton, or plutons, belonging to the Tiabaya Super-unit. The plutonics in the Ica region are summarised and compared to the Lima Segment. It is concluded that both segments have the same mode of outcrop in that sharply cross-cutting contacts are the rule and that the batholith was emplaced at the same high level in the crust by the same cauldron subsidence mechanism along the full length of Peru. There are however some differences between the segments. When the Arequipa Segment is compared to the Lima Segment it can be demonstrated to have been emplaced over a much shorter period, has no associated ring complexes and lacks the large plutons of monzogranite that are present in northern Peru. It also has an association between certain super-units (Linga and Tiabaya) and metalliferous, mainly copper, mineralisation, and such an association is lacking in the Lima Segment. Unconformably overlying the batholith are two formations of terrestrial volcanics of Tertiary age. These have been correlated on a regional scale and the Lower Formation is equivalent to the Calipuy Group of northern Peru and the Upper Formation is probably equivalent to the Nazca tuffs of southern Peru, although this has still to be confirmed
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.580691  DOI: Not available
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