New Geological, Petrological, and Geochemical Investigations of Alishar Volcanic rocks, NW of Saveh, Iran
Abstract
Two slivers of Eocene volcanic rocks, separated by Pliocene-Quaternary conglomerate, are exposed, northwest of the city of Saveh, along the Tertiary Uromieh-Dokhtar magmatic zone. The area north of Alishar, between 49o, 53'E and 50o,00'E longitudes and 35o, 17' N and 35o, 20' N latitudes, is located in the northern sliver. The oldest rocks north of Alishar have a Middle Eocene age, and are exposed in the core of an anticline which is faulted in its southern limb. These rocks are crystal and lithic tuff, and covered by rhyolitic ignimbrite. They are disconformably overlain by conglomerate, sandstone, marl, lenses of nummulite- bearing limestone, and green tuff. Fossils in the limestone give a Middle Eocene age. Bimodal, Late Eocene volcaniclastic and volcanic rocks cover these rocks. Evidence for acidic and intermediate volcanic activities is apparent at two levels. The early products of the explosive Late Eocene volcanic/volcaniclastic activity in this area were dacitic, and laid down on wet, green Middle Eocene tuff. These rocks are characterized by their vitroclastic texture and abundance of volcanic glass shard mixed with oxidized basic glass. The first level of basic-intermediate lavas is andesitic, with coarse clinopyroxene phenocrysts at the base, and plagioclase phenocrysts at the top, suggesting crystal separation at the scale of this unit. Extensive ignimbrites, including ignimbrite-breccia, ignimbrite-tuff, and ignimbrite-lava pairs cover this unit. The ignimbrites are rhyolitic, trachy-dacitic, dacitic, and trachy-andesitic. Mafic inclusions inside felsic groundmass are evidence for magma mingling in the ignimbrites and their compositional variation from rhyolite to trachy-andesite. The ignimbrites are cut by basaltic dikes and by pipes which probably were conduits for the movement of hydrothermal fluids. Epiclastic breccia and overlying level-2 andesitic lava cover the ignimbrite. These rocks are cut by masses of post-Eocene quartz diorite. Studies of the major, trace, and rare earth elements, show a wide range of variation in the felsic rocks and progressive differentiation from mafic to felsic rocks, and suggest a bimodal volcanism in the area north of Alishar. Primitive mantle- and chondrite-normalized spider diagrams show a moderate enrichment of HREE (with La/Sm=3.97-6.86 and La/Yb=6.11-12.11) and negative Nb and Ti anomalies, suggesting backarc basin volcanism. A comparison of the REE in the Alishar rocks with those in the upper crust, in addition to revealing similarities, indicates the enrichment in the LILE. It seems that during Middle and Late Eocene, intermediate- basic magma in the upper crustal levels, in addition to undergoing differentiation, mingled and mixed with the rhyolitic magma which formed from the melting of the rocks around the magma chamber.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V31C2161M
- Keywords:
-
- 3001 Back-arc basin processes;
- 5480 Volcanism (6063;
- 8148;
- 8450);
- 8413 Subduction zone processes (1031;
- 3060;
- 3613;
- 8170)