Abstract Knowledge of the granites of Sumatra has been gathered mainly as the result of systematic mapping programmes conducted with the aim of identifying mineral resources and providing a geological data base for more detailed studies. Mapping programmes were conducted principally by Dutch and Indonesian geologists prior to the second world war, mainly in southern Sumatra and the Tin Islands. In the 1970s a combined Indonesian Directorate of Mineral Resources (DMR)/British Geological Survey (BGS) project was set up to map the geology of Sumatra to the north of the Equator. On completion of this project in the mid-1980s geological and geochemical maps for the region were published at the scale of 1:250 000, together with descriptive sheet bulletins. Another useful compilation which may be refered to is the 1:2.5 million scale geological map for the whole of the Indonesian Archipelago which includes Sumatra (Clarke 1990). Subsequently BGS undertook a similar but smaller project in southern Sumatra in order to upgrade geological mapping and mineral exploration programmes which were being conducted by the Indonesian Geological Research and Development Centre (GRDC) and DMR. As part of this programme a specific effort was made to investigate the granites of this region. A combined granite workshop/regional mapping programme resulted in the identification of many granite units within batholiths such as Lassi, Bungo and Garba, as well as numerous isolated plutons. Full geochemical and isotopic analyses were provided for these granites (McCourt & Cobbing 1993; McCourt et al . 1996). Gasparon & Varne (1995) have provided further

Abstract The opening of the South Atlantic ocean at 110 Ma triggered the inversion of the Casma basin and the switch from marine volcanicity to plutonism, which evolved through three distinct phases. The first was the intrusion of the Coastal Batholith which forms a well-defined linear structure over the whole coastal region and which, in the Lima segment, endured from 100 to 60 Ma, and was terminated by the formation of the ring complexes. The second was the post Incaic development of the andesitic terrestrial plateau volcanics, the Calipuy group with associated scattered plutons of tonalite and granodiorite, which extended from perhaps 50 Ma to 20 Ma, and the third was the emplacement of the high level stocks and associated ignimbrite sheets from 20 to 6 Ma. Of these the Cordillera Blanca batholith which is of trondhjemitic affinity is the most important, and it is the intrusives of this zone which are the most important economically. Many plutons within the Batholith were emplaced into the brittle crust by processes of magmatic stoping and some of the evidence for this process is presented.

Cordilleran I-type granitoids with a mantle signature are characteristic products of oceanic-plate subduction at continental margins. I-type granitoids with a crustal signature occur in a variety of tectonic settings, including that of subduction. S-type granites with crustal signatures are characteristic of collisional settings, but also occur in the same range of settings as crustal I-types. The role of the tectonic setting is subordinate to that of the composition of the source region in determining the typology of crustal granites, which is a function of the proportion of mantle-derived to crustal material mobilized during magma genesis. The production of melts with similar proportions of mantle to crustal components is triggered by a variety of tectonic processes in different tectonic settings. Crustal heterogeneity is probably the main factor contributing to the diversity of crustal granites.

Segmentation of both the Lower Paleozoic and the Mesozoic orogenic belts of Peru is indicated by rapid variation in thickness and lithology of stratigraphic units and by the presence of lateral structures. Some of these segmental boundaries are common to both the Paleozoic and the Mesozoic fold belts and are probably old structures which have been episodically rejuvenated. The Coastal Batholith is also segmented compositionally, and some of the segment boundaries coincide with the orogenic segmentation. Others, however, do not. Those which do are probably deep-reaching, crustal-mantle faults which have penetrated sufficiently deeply into the source region of the granitoid magmas to affect both the composition of the magmas and the timing of magma generation within adjacent segments. A suite of early gabbros is common to the batholith as a whole, but the batholithic segments comprise granitoid super-units which are specific to the segments. Each super-unit may range from diorite to monzogranite, and the differences are established on the basis of textural features, the order of emplacement, and geochronology. Two batholithic segments have been mapped in detail. These two segments are compared, and it is shown that the Lima segment is the more complex, having no more than ten units and super-units which were emplaced between 100 and 30 Ma. The Arequipa segment consists of only 5 super-units which were emplaced between 100 and 80 Ma. A belt of Tertiary intrusives has also been distinguished to the east of the batholith which consists of numerous, small, scattered, plutons. The mainly Cretaceous plutonism of the batholith is spatially associated with thick sequences of pre-orogenic submarine lavas and volcaniclastic rocks which are the products of a volcanic arc constructed on thinned continental crust, whereas the Tertiary intrusives are associated with post-orogenic terrestrial plateau volcanics which were deposited on thickened crust. There is some evidence that the Cretaceous and Tertiary lavas are isotopically different. In terms of metal association, the Cretaceous belt is a copper, molybdenum, and gold metallogenic province, whereas the Tertiary belt is polymetallic, producing mainly copper, lead, zinc, and silver.