Origin of the island arc Moho transition zone via melt-rock reaction and its implications for intracrustal differentiation of island arcs: Evidence from the Jijal complex (Kohistan complex, northern Pakistan)

Abstract

If the net flux to the island arc crust is primitive arc basalt, the evolved composition of most arc magmas entails the formation of complementary thick ultramafic keels at the root of the island arc crust. Dunite, wehrlite, and Cr-rich pyroxenite from the Jijal complex, constituting the Moho transition zone of the Kohistan paleo–island arc (northern Pakistan), are often mentioned as an example of high-pressure cumulates formed by intracrustal fractionation of mantle-derived melts, which were later extracted to form the overlying mafic crust. Here we show that calculated liquids for Jijal pyroxenites-wehrlites are strongly rare earth element (REE) depleted and display flat or convex-upward REE patterns. These patterns are typical of boninites and are therefore unlike those of the overlying mafic crust that have higher REE concentrations and are derived from light rare earth element (LREE)–enriched melts similar to island arc basalt. This observation, along with the lower 208Pb/204Pb and 206Pb/204Pb ratios of Jijal pyroxenites-wehrlites relative to gabbros, rejects the hypothesis that gabbros and ultramafic rocks derive from a common melt via crystal fractionation. In the 208Pb/204Pb versus 206Pb/204Pb diagram, ultramafic rocks and gabbros lie on the same positive correlation, suggesting that their sources share a common enriched mantle 2 (EM2) signature but with a major depleted component contribution for the ultramafic rocks. These data are consistent with a scenario whereby the Jijal ultramafic section represents a Moho transition zone formed via melt-rock reaction between subarc mantle and incoming melt isotopically akin to Jijal gabbroic rocks. The lack in the Kohistan arc of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks implies either (1) that a substantial volume of such ultramafic cumulates was delaminated or torn out by subcrustal mantle flow from the base of the arc crust in extraordinarily short time scales (0.10–0.35 cm/yr), or (2) that the net flux to the Kohistan arc crust was more evolved than primitive arc basalt

Yin and yang of continental crust creation and destruction by plate tectonic processes

“…Greene et al (2006) calculated that more than 25 wt% of Talkeetna primary melts crystallized as pyroxenite at the base of the crust but were mostly lost, probably together with dense garnet granulites, foundering into the underlying mantle. Similar observations have been made for the Cretaceous Kohistan intra-oceanic arc of the Pakistani Himalaya, where the lack of cogenetic ultramafic cumulates complementary to the evolved mafic plutonic rocks led Garrido et al (2007) to conclude that either such cumulates formed but were lost from the base of the arc crust or that the net flux to the Kohistan arc crust was more evolved than primitive basalt.…”

Mountain Building, Tectonic Evolution, Rheology, and Crustal Flow in the Himalaya, Karakoram, and Tibet

“…The Kohistan arc comprises a complete upper mantle and crust section through an island arc (Figure 7). The mantle component includes the ultramafic lower part of the Jijal complex (layered dunite, wehrlite, and Cr-rich pyroxenites overlain by websterites and pyroxenites; Dhuime et al, 2007;Garrido et al, 2007) and the Sapat ultramafic (dunite and harzburgite) thrust slices. The crustal component includes the upper part of the Jijal complex, Kamila complex (garnet granulites and basal amphibolites of MORB affinity), Chilas complex (gabbronorites and diorites), and overlying andesitic-dacitic volcanic complexes (Dir, Utror, Shamran, and Chalt volcanic suites).…”

Andesites as Magmatic Liquids or Liquid-crystal Mixtures; Insights from Egmont and Ruapehu Volcanoes, New Zealand Research Article

“…The inferred crustal structure beneath Taranaki is consistent with that exhibited by exhumed fossil arc systems, e.g. Kohistan in northern Pakistan [3,26,27,[48][49][50][51] and Talkeetna in Alaska [52,53] which are thought to be oceanic arcs. These long-lived arcs developed a crustal sequence of ultramafics across the crust/mantle boundary, overlain by thick cumulate gabbros and norites, mid-crustal mafic to siliceous intrusives and supracrustal intermediate to acid volcanics.…”