Geology Reference
In-Depth Information
Figure 7.4
Contact between western margin of the Santa Rosa Tonalite, Coastal
Batholith Peru and country rock comprising marine basalts and basaltic and-
sites. The contact is knife sharp and lacking in significant internal deformation,
suggesting a passive mode of emplacement. The tabular intrusion is cut by syn-
magmatic mafic sills and dykes that extend into the country rock.
7.3 Zoned Plutons
Zonation is seen in batholithic complexes worldwide. In the Coastal Batholith
of Peru the overall composition of different plutons varies from diorite to
leucogranite (generally low colour index granites with high silica content circa
70% wt. %). Although abrupt changes in rock type are usually confined to
the often knife-sharp contacts between plutons, there are also less prominent
variations, particularly within plutons. One excellent example of pluton
zonation that has been mapped in detail is the circa 90 Ma old Tuolumne
Meadows pluton in the Sierra Nevada batholith, USA (Figure 7.6). Here
an outer facies of tonalite/granodiorite grades inwards into leucogranite and
granite porphyry (Section 7.7.1).
Where, as in the case of Tuolumne Meadows, the composition of the pluton
changes from mafic at its margins to more acidic in the core, the zonation is
referred to as normal. The reverse, although less common, may also occur (e.g.
Smartville Complex, California).
