Abstract The Molucca Sea Collision Zone in eastern Indonesia is the site of an orthogonal collision between two active subduction systems. Both the Halmahera subduction zone, to the east, and the Sangihe subduction zone, to the west, have subducted oceanic lithosphere of the Molucca Sea Plate, which has now been completely consumed. Both volcanic arcs were active since the Neogene and provide a means of probing the element fluxes through the two systems. The geochemistry of Neogene and Quaternary lavas from each volcanic arc is compared to constrain changes in the mass fluxes through the systems and the processes controlling these fluxes at different times during their history. Both arcs show increased evidence for sediment recycling as the collision progressed, but for contrasting reasons. In Halmahera this may represent an increased sediment flux through the arc front, while in Sangihe it may simply reflect a greater opportunity for melting of sediment-fluxed portions of the mantle wedge. In both cases the change in arc geochemistry can be related to the evolving architecture of the particular subduction zone. The Halmahera lavas also record a temporal change in the chemistry of the mantle component that resulted from induced convection above the falling Molucca Sea Plate drawing compositionally distinct peridotite into the mantle wege.

Abstract Sm-Nd garnet geochronology is often hampered by the presence of submicroscopic inclusions of rare earth element-rich phosphates, which lower age precision, lead to inaccurate ages or make dating impossible. We propose a single-step sulphuric acid leaching technique as a very efficient tool in eliminating phosphate inclusions, which helps to achieve more precise and more accurate Sm-Nd garnet dates. Examples from silimanite grade metapelites demonstrate the much higher efficiency of this method in comparison with previously proposed techniques based on HF and HCl. 147 Sm/ 144 Nd ratios obtained on garnets leached by sulphuric acid were twice as high as those obtained by HF and HCl leaching. This led to age precision better than 3% for Tertiary samples. Comparison of leached and unleached nearly inclusion-free garnets from high pressure granulites, demonstrates that there is no Sm/Nd fractionation induced by H 2 SO 4 leaching. Our new technique eliminates phosphates, but does not attack garnet. This considerably reduces the necessity for hand-picking and lowers the amount of sample required for analysis making Sm-Nd garnet dating a more easily applied geochronometer.

The Chilas Complex is a large mafic-ultramafic body closely associated with the Kohistan Arc sequence in the western Himalaya of northern Pakistan. The arc and the Chilas Complex occupy an area of 36,000 km 2 , bounded on the north and south by major sutures. The arc formed close to the margin of Eurasia in response to the northward subduction of neo-Tethyan ocean lithosphere in Late Jurassic to middle Cretaceous time, and consists of intra-arc sediments, calc-alkaline volcanics, and diorite-tonalite-granite plutons. At its base is the Chilas Complex, which extends for more than 300 km and which has a maximum width of 40 km. Most of the complex consists of massive (although locally layered) gabbro-norites, which comprise variable amounts of plagioclase (An 64-40 ), orthopyroxene (En 76-48 ), clinopyroxene (mg = 75-55), magnetite, ilmenite, ±quartz, ±K-feldspar, ±hornblende, ±biotite, ±rare scapolite. In the central part of the complex, near the base, there are minor discordant dikes and intrusive bodies as large as 5 km 2 of a dunite-peridotite-troctolite-gabbronorite-pyroxenite-anorthosite association that displays excellent layering, graded bedding, slump breccias, and syndepositional faults. These rocks contain olivine (Fo 94-71 ), relatively Mg-rich orthopyroxene (En 91-65 ), clinopyroxene (mg = 85-67), and calcic plagioclase (An 98-83 ), ±hornblende, ±chrome spinel, and ±pleonaste, and represent a more primitive magma batch emplaced into the base of the gabbro-norite magma chamber. The mafic complex is not an ophiolite. Rocks of the complex have more petrographic and compositional similarities with plutonic blocks from island arcs and with other major mafic complexes such as the Border Ranges Complex of Alaska and those from the Ivrea Zone in the Alps. Trace-element patterns of the gabbro-norites have marked negative Nb anomalies, positive Sr, Ba, and P anomalies, and high K/Rb ratios, features consistent with melting of a hornblende-bearing sub-arc mantle source. The Chilas Complex either represents the root zone magma chamber of the Kohistan island arc, or magma generated by diapirism in the early stages of intra-arc rifting during formation of a back-arc basin.