Partial melting of lower crust at 10-15 kbar: constraints on adakite and TTG formation

Contrib Mineral Petrol (2013) 165:11951224

DOI 10.1007/s00410-013-0854-9

Partial melting of lower crust at 1015 kbar: constraints on adakite and TTG formation

Qing Qian Jrg Hermann

Received: 21 September 2012 / Accepted: 12 January 2013 / Published online: 3 February 2013 Springer-Verlag Berlin Heidelberg 2013

Abstract The pressuretemperature (PT) conditions for producing adakite/tonalitetrondhjemitegranodiorite (TTG) magmas from lowercrust compositions are still open to debate. We have carriedout partial melting experimentsofmac lower crust in the piston-cylinder apparatus at 1015 kbar and 8001,050 C to investigate the major and trace elements of melts and residualminerals and further constrain the PT range appropriate for adakite/TTG formation. The experimental residues include the following: amphibolite (plagioclase ? amphibole garnet) at 1015 kbar and 800 C, garnet granulite (plagioclase ? amphibole ? garnet ? clinopyroxene ? orthopyroxene) at 12.5 kbar and 900 C, two-pyroxene granulite (plagioclase ? clinopyroxene ? orthopyroxene amphibole) at 10 kbar and 900 C and 1012.5 kbar and 1,000 C, garnet pyroxenite (garnet ? clinopyroxene amphibole) at 13.515 kbar and 9001,000 C, and pyroxenite (clinopyroxene ? orthopyroxene) at 15 kbar and 1,050 C. The partial melts change from granodiorite to tonalite with increasing melt proportions. Sr enrichment occurs in partial melts in equilibrium with \20 wt% plagioclase, whereas depletions of Ti, Sr, and heavy rare earth elements

(HREE) occur relative to the starting material when the amounts of residual amphibole, plagioclase, and garnet are [20 wt%, respectively. Major elements and trace element patterns of partial melts produced by 1040 wt% melting of lower crust composition at 1012.5 kbar and 800900 C and 15 kbar and 800 C closely resemble adakite/TTG rocks. TiO2 contents of the 1,0001,050 C melts are higher than that of pristine adakite/TTG. In comparison with natural adakite/ TTG, partial melts produced at 1012.5 kbar and1,000 C and 15 kbar and 1,050 C have elevated HREE, whereas partial melts at 13.515 kbar and 9001,000 C in equilibrium with [20 wt% garnet have depressed Yb and elevated La/Yb and

Gd/Yb. It is suggested that the most appropriate PT conditions for producing adakite/TTG from mac lower crust are 800950 C and 1012.5 kbar (corresponding to a depth of 3040 km), whereas a depth of [4550 km is unfavorable.

Consequently, an overthickened crust and eclogite residue are not necessarily required for...