batholith

Batholiths are complex intrusive bodies composed of plutonic igneous rocks, usually of felsic or intermediate rock-types, such as granite, quartz monzonite, or diorite. Batholiths are also called granite domes.

The exposed surface area is defined (by geographers) as more than 100 square kilometers, though large geological batholiths may have less exposure. Areas smaller than 100 square kilometers are called stocks. The entire batholithic emplacement is typically so large that the bases are rarely exposed, and batholiths have steeply inclined walls that form prominent dome structures when exposed by erosive removal of initially overlying rocks.

Sometimes batholiths arise through several smaller diapiric intrusions (plutons) and have a complex history of magmatic intrusion and crystallization at depths of 5 to 30 kilometers. Batholith formation is commonly associated with lithospheric plate boundaries, where tectonic interactions between plates are associated with large scale melting of crustal rocks and the formation of deep magma chambers. As erosion uncovers the crystalline rock that formed at great depth, crystal structures respond to the decrease in load and expand, rendering the plutonic rocks susceptible to exfoliative weathering.

Spheroidal weathering produces boulder fields, and sheet-like exfoliative weathering, which is accelerated by frost wedging, creates smooth structures like Half Dome in the 40,000 sq km Sierra Nevada Batholith in Yosemite (upper left, right - click to enlarge). The Coastal Plutonic Complex of British Columbia and Alaska is even larger, extending for 1,800 kilometers and covering 182,500 sq km. Other North American batholiths include the following: The Idaho Batholith is a composite mass of granitic plutons covering approximately 15,400 square miles in central Idaho [map, Castle Peak, Contact Batholith]. Guichon batholith in British Columbia contains several large, low-grade copper deposits. The South Mountain Batholith (SMB) of southwestern Nova Scotia is the largest granite batholith in the Appalachian Orogen with an approximate area of 7300 sq km.

The Cornubian Batholith was intruded into south-west England at the close of the Variscan Orogeny (late Carboniferous - Permian).

[images: photos: Baja Batholith spheroidal weathering, Boulder Batholith & spheroidal weathering, Conrnubian batholith Cligga Head, Enchanted Rock batholith & tree at summit, Golden Horn Batholith, Rocky Mtn. Nat Park, Yosemite batholith, batholith, outcrop Granite Mountain; websites: Avalonia, Halifax Pluton, Boulder Batholith, Cornubian Batholith, Peninsular Ranges batholith, Sierra Nevada Batholith & lithography; lithography: conness contact, Enchanted Rock granite, 2, Julianehåb batholith granite, gallery, mingling of microdioritic and granodioritic magmas in the calc-alkaline Pataz Batholith of the northern Peruvian Cordillera Oriental, schlieren banding in the Sausfjellet pluton, rock gallery including Vioolsdrif Batholith; maps: Boulder Creek Batholith, Cornubian Batholith, Enchanted Rock Batholith, Halifax Pluton, Idaho batholith ecoregions, Verkhisetsk batholith, batholiths in Wisconsin; diagram: plutons and volcanic landforms

| 0 Guide-Glossary

xenoliths

Xenoliths, meaning 'foreign stones', arise when fragments of metamorphically altered country rock fall into magma or lava and become enveloped within igneous rock. Xenoliths can be engulfed along margins of magma chambers, torn loose from the walls of a conduit for erupting lava or an explosive diatreme, or picked up along the base of flowing lava. Xenoliths can be quite large and xenoliths can occur in clusters within the plutonic rock or demonstrate dikelet intrusions by the magma.

Xenoliths are distinguished from paleosomes, which are older bodies comprising refractory minerals that failed to melt within migmatites (in which more fusible neosomal minerals melted during Barrovian regional metamorphism).

[images : xenolith of peridotite embedded in vesicular basalt?; olivine in peridotite xenolith in alkali basalt; xenolith of partly melted metamorphic rock embedded in solidied lava; xenolith embedded in granite, Garnet Canyon, Tetons, WY; xenolith in pink granite, 2; xenolith in granodiorite, and close-up of boundary; xenolith of biotite-rich schist enclosed in Petersburg Granite; xenolith; xenolith in a basaltic sill; spinel lherzolite xenoliths, San Carlos AZ; xenolith of spinifex-textured basalt within vesicular dacite glass; altered xenolith of spinifex rock in altered and veined dacite; xenoliths in boulder; Shiprock xenolith; formations: xenolith of a quartz-biotite gneiss into which granite was intruded; xenolith in granite quarry, Elberton, GA, 2; excellent, though large bandwidth, illustrations of a xenolith in the Halifax Pluton]