Thomas A. Vogel

Major-element chemistry of plutonic rock suites from compressional and extensional plate boundaries☆

Chemical criteria have been developed to distinguish plutonic rock suites from compressional and extensional plate margins by comparing type examples of Mesozoic and Cenozoic plutonic suites generated at these plate boundaries. Compressional plutonic rock suites are characterized by intermediate unimodal frequency distributions of differentiation index and normative plagioclase, calc/alkali indexes in the range of 60–64, and distinctive patterns on AFM diagrams. Extensional plutonic rock suites are characterized by bimodal distribution of differentiation index and normative plagioclase, calc/alkali indexes in the range of 50–56 and the presence of peralkaline rocks. These criteria are useful in determining tectonic settings of plutonic rock suites of unknown environment. No single criterion should be used to distinguish tectonic setting. The distinguishing chemical features of compressional plutonic rock suites may directly be related to melting and dehydration of the subducted ocean crust. Other processes that may be important are: lowering of the solidus of the overlying peridotitic mantle wedge resulting in partial melting; fractionation of both primary melts; extensive reaction of these melts with the continental crust. The characteristics of extensional plutonic rock suites may directly be related to melting of anhydrous peridotitic mantle; small amounts of melting of continental crust and a lack of mixing of the two magmas. Basic rocks from extensional suites may be generated by smaller amounts of melting at greater depths than those from compressional suites resulting in some with alkaline affinities.

Rare-element–enriched, S-type ash-flow tuffs containing phenocrysts of muscovite, andalusite, and sillimanite, southeastern Peru

Ash-flow tuffs of Neogene age exposed over 2,500 km 2 in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87 Sr/ 86 Sr i (0.7258 and 0.7226) and δ 18 O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.” Mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a HF/ a H 2 O. The chemical, isotopic, and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock.

COEXISTING ACIDIC AND BASIC MELTS: GEOCHEMISTRY OF A COMPOSITE DIKE'

Composite acidic-basic dikes near Winnsboro, South Carolina, consist of cuspate pillow-like masses of basic rock surrounded by granite with no chilled margins or gradational boundaries. These dikes are similar to the net-veined complexes observed in other parts of the world. Most workers agree that these complexes represent coexisting acidic and basic liquids. Explanations of origin fall into two general classes: liquid immiscibility or commingling of miscible liquids. In the first, a homogeneous magma has unmixed to form two contrasting magmas. In the second, mixing was prevented because of high viscosities and rapid crystallization. The origin of these coexisting magmas in the second case has been variously explained by: coincidental intrusion of magma from different sources; melting of acidic rocks by the basic magma; or successive fractional melting of a common parent. These models are tested in the Winnsboro composite dike on the basis that the light REEs are partitioned in favor of the liquid in liquid-crystal equilibrium. In the Winnsboro dike, the light REEs are enriched in the basic rocks and relatively depleted in the granitic rocks. Thus, fractional crystallization of the basic rock or partial melting of the wall rock must be rejected.

Calcic cores of plagioclase phenocrysts in andesite from Karymsky volcano: Evidence for rapid introduction by basaltic replenishment

Calcic cores in plagioclase of Karymsky andesite of the 1996–2000 eruptive cycle texturally and compositionally (both trace and major elements) mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle. These observations support the view that simultaneous eruption of andesite and basalt at Karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir. Homogeneity of andesitic output occurred within two months. This suggests to us that blending of injected basalt into reservoir magma was thorough and rapid.

The Composite Dikes at Mount Desert Island, Maine: An Example of Coexisting Acidic and Basic Magmas

Composite dikes of Mount Desert Island are composed of pillow-like masses of basalt surrounded by granite and represent coexisting acidic and basic liquids. The contacts between basalt and granite ate sharp on both a megascopic and microscopic scale. Grain-size reduction at basalt pillow margins is generally present and is due to chilling. Evidence of commingling consists of quartz and microcline xenocrysts in the basalt as well as hybrid rocks formed by incomplete mixing of basalt and granite. Rare earth elements in experimental immiscible systems have been shown to be highly partitioned towards the basic liquid (

Constraints on magma ascent, emplacement, and eruption: Geochemical and mineralogical data from drill-core samples at Obsidian dome, Inyo chain, California

D_{b/g} \sim 10

Chemical Mass Balance of the Earth's Crust: The Calcium Dilemma (?) and the Role of Pelagic Sediments

). In the Mount Desert Island complex, however, the light REEs are enriched in the granite with