In:
Geological Magazine, Cambridge University Press (CUP), Vol. 126, No. 5 ( 1989-09), p. 515-547
Abstract:
At least 2000 km 3 of relatively uniform dacitic magma have been erupted from the Cerro Galan caldera complex, northwest Argentina. Between 7 and 4 Ma ago several composite volcanoes predominantly of dacitic lava were constructed, and several large high-K dacitic ignimbrites were erupted. 2.2 Ma ago the 〉 1000km 3 Cerro Galan ignimbrite was erupted. The predominant mineral assemblage in the ignimbrites is plagioclase-biotite-quartz-magnetite-ilmenite; the Cerro Galan ignimbrite also contains sanidine. Fe-Ti oxide minerals in the Cerro Galan ignimbrite imply temperatures of 801–816 °C. Plagioclase phenocrysts in the ignimbrites typically have rather homogeneous cores surrounded by complex, often oscillatory zoned, rims. Core compositions show a marked bimodality, with one population consisting of calcic cores surrounded by normally zoned rims, and a second of sodic cores surrounded by reversely zoned rims. The older ignimbrites do not show systematic compositional zonation, but the Cerro Galan ignimbrite exhibits small variations in major elements (66–69% SiO 2 ) and significant variations in Rb, Sr, Ba, Th and other trace elements, consistent with derivation from a weakly zoned magma chamber, in which limited fractional crystallization occurred. The ignimbrites have 87 Sr/ 86 Sr = 0.7108–0.7181; 143 Nd/ 144 Nd = 0.51215–0.51225, and δ 18 O = + 10 to + 12.5, consistent with a significant component of relatively non-radiogenic crust with high Rb/Sr and enriched in incompatible elements. Nd model ages for the source region are about 1.24 Ga. 87 Sr/ 86 Sr measurements of separated plagioclases indicate that Anrich cores have slightly lower 87 Sr/ 86 Sr than less calcic plagioclases, suggesting a small degree of isotopic heterogeniety in different components within the magmas. Pb isotope data for plagioclase show restricted ranges ( 206 Pb/ 204 Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb = 18.87–18.92, 15.65–15.69 and 39.06–39.16 respectively), and suggest derivation from Proterozoic crustal material( 〉 1.5 Ga). Contemporaneous satellite scoria cones and lavas are high-K basalts, basaltic andesites and andesites with SiO 2 = 51–57%; K 2 O = 2–3% and normative plagioclase compositions of An 37–48 , and may be derived from a mantle source containing both ‘subduction zone’ and ‘within plate’ components. 87 Sr/ 86 Sr ranges from 0.7055 to 0.7094 and 143 Nd/ 144 Nd from 0.51250 to 0.51290. Variation diagrams such as MgO: SiO 2 show two trends, one indicating closed system fractional crystallization and the other crustal contamination. AFC modelling of the open system rocks indicates a parental mantle-derived mafic magma which is itself enriched in K, Rb, Ba, U, Ta/Sm, Ta/Th and Sr, and has 87 Sr/ 86 Sr = 0.705–0.706, while the contaminant need not be more radiogenic than the dacitic ignimbrites. The Cerro Galan dacitic magmas are interpreted in terms of a deep and uniform region of the central Andean continental crust repeatedly melted by emplacement of incompatible-element-enriched, mantle-derived mafic magmas, a proportion of which may also have mixed with the dacite magmas. A component of the crustal material had a Proterozoic age. The magmas derived by crustal melting were also enriched in incompatible elements either by crystal/liquid fractionation processes, or by metasomatism of their source regions just prior to magma generation. Much of the crystallization took place in the source region during the melting process or in mid-crustal magma chambers. The magmas may have re-equilibrated at shallow levels prior to eruption, but only limited compositional zonation developed in high-level magma chambers.
Type of Medium:
Online Resource
ISSN:
0016-7568
,
1469-5081
DOI:
10.1017/S0016756800022834
Language:
English
Publisher:
Cambridge University Press (CUP)
Publication Date:
1989
detail.hit.zdb_id:
956405-6
detail.hit.zdb_id:
1479206-0
SSG:
13
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