Geology Reference
In-Depth Information
several decades different groups of igneous geologists have sought to classify
calc-alkaline granitoids into tectonically-related groups using a range of field,
petrographic and geochemical criteria. The result has been a proliferation of
classification schemes using letters as shorthand for defining a particular typol-
ogy the most famous and widely known being the
I-S
classification of Chappell
and White. One area of outstanding contention remains the degree to which,
once classified, the granite type reflects its original magma source region. It
has proved possible to distinguish, using field criteria, between
I-type
(igneous
source) biotite - or hornblende-biotite granitoids and
S-type
(sedimentary
source) muscovite-biotite, or two-mica, granites. Other members of the granite
'alphabet soup' include A-types (alkaline) and M-Types. One important sub-set
of the granitoid family that does stand alone on age and/or compositional
grounds are the trondhjemite-tonalite-granodiorite (TTG) suites found mainly
in ancient, archaean crust. Trondhjemite is a name used for leucotonalites
(cf. Figures 3.11 and 3.12, Chapter 3) dominated by quartz and sodium-rich pla-
gioclase feldspar in which alkali feldspar is less than 10% of the total feldspar
and the colour index is less than 10. There are however phanerozoic examples
from modern subduction settings. The origin of these analogous, high sodium
plutonic rocks, whose associated extrusive magmas are commonly referred to
as adakites, originate via partial melting of amphibolite or garnet-amphibolite
protoliths. Although some granitoids have transitional characteristics, and others
may be difficult to designate because of hydrothermal alteration, the information
in Table 7.1 provides a basis for preliminary classification in the field.
The importance of calc-alkaline granitoids in continental margin arcs (e.g.
those of the circum-Pacific region) implies a magma-genetic link with subduc-
tion processes at destructive plate margins (cf. Chapter 1). About 100 000 km
2
of imperfectly exposed Mesozoic and Tertiary batholith forms the mountain
chains of the western Americas from Alaska to south Chile. Although much
of this batholith is unstudied, detailed fieldwork in some areas has revealed
considerable internal variation. Mapping of the Peru coastal batholith based on
careful tracing of internal intrusive-intrusive rock contracts within the batholith
over large distances, in this case made possible in the vertical dimension by the
deeply-dissected nature of the Andes has led to the map shown in Figure 7.2.
This is a geological map of a 120
×
70 km area showing some of the major
lithotypes (or units) comprising the batholith. The succession of emplacement
is from basic to acid with time meaning the plutons comprising the batholith
were constructed piecemeal from relatively small discrete magma batches.
Also in this area the batholith is cut by several major high-level ring com-
plexes and other minor shallow intrusions (cf. Chapter 6), comprising granites,
tonalites, diorites and gabbros that together confirm the link between intrusive
and extrusive activity during major batholith construction. At high altitudes,
the roof contacts of the batholith with overlying volcanics have been studied,
