Geology Reference
In-Depth Information
Chimbote - Huallanca
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Casma - Huaraz
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Pativilca - Conococha
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height
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S
distance (km)
Figure 7.5
Geophysical cross section across part of the Peruvian Coastal
Batholith based on gravity surveys. The plutons comprising the batholith (e.g.
Santa Rosa Tonalite, Figure 7.4) are revealed as thin (circa 1-4 km thick) sheets,
making the overall geometry of the batholith broadly tabular in form. The thicker
subsurface zone in contact with basaltic volcanic rocks (v) is interpreted as a
vertical magma feeder zone.
Normal zonation may result from
fractional crystallisation
processes
whereby, during crystallisation, early-formed high-temperature minerals (for
example, Ca-rich feldspars, Mg-rich pyroxenes or amphiboles) accumulated
in the solidifying walls of the intrusion, leaving residual liquids towards the
centre of the intrusion enriched in silica and alkali elements. In this way, a
primary granodiorite magma, for example may fractionate to form a range of
rock-types, from diorite and tonalite intrusion margins to granite at the centre,
and the variation in rock-types across such intrusions may be completely
gradational. As highlighted in the introduction, the identification of the intrusive
sequence depends on careful field mapping of the interiors of plutons, for if an
intrusion of diorite were invaded by a younger granite magma, then the best
way of reaching this interpretation in the field would be by finding internal
intrusive contacts (Section 7.5).
