N. Jb. Miner. Abh. 2009, Vol. 186/1 , p. 79-93, Stuttgart, July 2009, published online 2009 @ by E. Schweizerbart'sche Verlagsbuchhandlung 2009 Mixing and mingling of mafic and felsic magmas along the Neo-Tethys continental margin, Sanandaj-sirjan zone, Jalil Ghalamghash, Hassan Mirnejad and Hamideh Rashid With 7 figures and 4 tables Abstract. The Alvand pluton in northwest Iran consists of gabbro and granite units. Along the interface between granite and gab- bro, numerous ma{ic enclaves (ME) are concentrated. This interface is marked by a variety of hybridization and mingling fabrics/ textures including lobate/ellipsoid ME with chilled margins, back-veining granite, feldspar megacrysts in ME, and crenulated margins in enclaves. The chemical composition of the ME (SiO, = 45.04-52.07 wt%) has more affinity towards gabbro (SiO, = 45.01-46.00 wtTo) than grairite 1SiO. ='11.62-75.18 wtTo), indicating that the ME are rhe result of mingling of the gabbroic melt with a granitic magma. Diffusion reactions as the source of the ME are ruled out because the abundances of immobile elements such as Ni and Cr (37-144 ppm and 60-3 16 ppm) in the ME differ from those of the gabbro (446-5 I 8 ppm and,977 -1297 ppm). Therefore, it is possible that mixing of gabbroic and granitic melts, although to a minimal extent, was also involved in the genesis of the ME. Geochemical features of gabbro such as high Ni and Cr abundances suggest that it crystallized from a primary mantle- derived magma. In contrast, the peraluminous character of the granite and the presence of sedimentary and micaceous enclaves reflect a crustal origin for this rock type. Because the ME are found mainly around the contact between granite and gabbro, they likely formed at the place of final emplacement when granitic magma intruded the still unsolidified gabbro. At this stage, mingling and minor mixing processes led to the formation of melts similar to those of the ME. Such melts rose rapidly through a viscous granitic magma and chilled against it to form the ME scattered in the granite. Key words: Alvand, Iran, mafic enclaves, mingling, mixing, Sanandaj-Sirjan zone. NW lran: A case study from the Alvand pluton Introduction The Sanandaj-Sirjan zone is a NW-SE striking unit (Fig. 1a) that belongs to the 1500-km-1ong Zagros oro- genic system, between the Urumieh-Dukhtar magmatic arc and theZagros fold-thrust belt (Alavi 1994).TheZa- gros orogenic system, Iranian part of the Neo-Tethyan orogenic collage, was formed by norlheastward subduc- tion of the Neo-Tethyan oceanic lithosphere and subse- quent continental collision between the Iran and Afro- Arabian Plates in Mesozoic to Cenozoic time (DnwEv et al. 19'73, BsnsrnraN & KrNc 1981, Rrcou 1994, Allvr 1994). The Sanandaj-Sirjanzone is also characterized by highly deformed metamorphic rocks which are associated with abundant deformed and undeformed plutonic and volcanic rocks (MouAJJEL & FnncussoN 2000). Intru- sions are well developed in the northern parts of the zone where they form a long belt from northwest (Urumieh) to DOI: 1 0. 1 1 27 /OO77 -7 7 57 /2O09/O1 33 southeast (Golpayehgan) for about 800 km (Fig. 1b), and consist dominantly of mafic and felsic rocks. Because the rocks of the Urumieh-Golpayegan plutonic belt are wide- spread, diverse and fresh, they provide a unique oppor- tunity to study the patterns of plutonims along the Neo- Tethys active continental margin. Previous studies on the geology and petrogenesis of Urumieh-Golpayehgan plutonic belt suggest that it formed in an active continental margin setting (BnnennrlN & B BnsBnrA.N 1 98 1, InaNr 1992, Y rrtz xlsH & SnoecHrA.N 1996, GSaUMGHASH 2002, GHII-AMGHASH et al. 2003, Acanr et al. 2005, AHNI.q.Ir-KHALADJ et aL.2001 ,Mnz- HARy et al. 2008, Omn,q.Nl 2008, Snpnru 2008, Tomr.q.r.{ et al. 2008). Gna.r.qucua.sH et al. (2003), Gnereucuasu et al. (2005), MazHARy et al. (2008) and TonrraN et al. (2008) point out the interaction zone between granite and gabbro developed in the Urumieh, Alvand, Piranshahr and Qorveh plutons. However, many of these studies did o20oeE.schweizerbart'scn"v",hgg?lf;u7n1fl li:S1rt1?8.E,f, #