Island-arc magma sources and island-arc volcanism evolution.
Geological department of Moscow State University
New petrologic and geodynamic model for island arc systems is proposed to satisfactory explain of space and spatial variations of island arc volcanic series. The main idea of the model is step-by-step involving of additional sources of melting to magma generation processes during evolution of the island arc system. Each stage of the island arc evolution is characterized by the specific set of volcanic series.
Primitive island arcs are defined by one main source of magma. Magma generated by fluid induced melting of the mantle wedge above subducted slab. As a result, Hi-Mg and Low-K basalts are predominant volcanic rocks in primitive island arcs. It has clear island arc geochemical signature which produced by fluid-mobile influx from the slab.
The thickness of the island arc crust is growing up during island arc evolution and magmas could be more evolved with island arc crust growth. This crust is consists of volcanogenic material, which usually altered to greenschists. Global scale of such low-grade metamorphism is proved by occurrence of wide distributed few-million years old island arc volcanic rocks, which mostly metamorphized. These greenschists could be easily transformed to amphibolites in lower parts of island arc crust. Thus, after several million years primitive island arc became a developed island arc with crust which consists of rocks metamorfized in greenschists and amphibolite facies.
Large volume of silica-rich and intermediate volcanic rocks is common for developed island arc. It may be explained by additional new area of magma generation inside the crust [Tamura,Tatsumi,2002; Dufek, Bergantz,2005]. Partial melting of greenschists produce silica-rich melts [Montel,Vielzeuf,1997], amphibolites could produce silica-rich melts at the low pressure [Johannes,Holtz,1996; Nakajima, Arima,1998; Lupulesku, Watson,1999] or could be “granitized” [Selbekk et al.,2002]. Experiments [Rapp,Watson,1995; Gardien et al.,2000] and calculations [Kimura et al.,2002] shows that partial melting of amphibolites in water-saturated conditions and at 8-10 kbar of the pressure (it’s corresponds to lower parts of developed island arcs) could produce andesibasalts or even basalts (at high degree of the partial melting). It’s important, that amphibole is a restite phase at these conditions. If we assume that the lower crust amphibolites are formed from high-Mg basalts of previous stage, it leads to producing of less magnesian melts than high-Mg basalts of primitive island arcs. Amphibole-bearing restite could preserve significant amount of LREE, Nb, Ti and K. As a result of melting of the island arc lower crust we can expect magmas which are well corresponding to low-K tholeitic island arc series of volcanic fronts of developed island arcs.
I suppose that magmas could emanate simultaneously from several levels of the island arc system: 1) high-Mg low-K basalts could be generated by fluid-induced melting of the mantle wedge; 2) low-K andesibasalts and basalts are from ampibolite melting at the lower crust conditions; 3) silica-rich magmas could be formed by melting of island arc upper crust metamorphic rocks. All these magmas could mixing each other in transitional magma chambers and then erupt in the same volcanic center with hybrid rocks forming. Such scheme is a very confusing factor for clear determination of volcanic series for a lot of island arc volcanoes.
Mature island arcs (like Japan or Kamchatka) are developing after jump of a subsuction toward to ocean. Such jumps are very common for most of known island arcs and responsible for two-chain structure of island arcs. Volcanic front starts to form again after such jump following the scenario which described above, whereas former volcanic front shifts to backarc settings and could suffer farther evolution. Fluid induced melting is impossible at this stage and could be only as relics. At the moment the island arc crust under forming volcanic front is consist of metavolcanic rocks in upper part and restites of melting in lower part of the crust. As was concerned above, melting at lower crust settings could leads to amphibole enrichment in restites and after dehydratation will form pyroxenites or amphibole-bearing pyroxenites. P.Kelemen and coauthors [Kelemen et al., 2003] demonstrated occurrence of pyroxenites in lower part of the palaeoarc Talkeetna (Alaska) and shows with mass-balance that significant amount of pyroxenites were delaminated. If these pyroxenites were formed as restites after producing of low-K volcanic front magmas, it means that its composition will be complementary with low-K magmas, i.e. pyroxenites will be enriched in K, Ti, Nb, LREE in comparison with primary amphibolites, which formed after primitive island arc basalts. [Jull,Kelemen, 2001] demonstrated that pyroxenitic lower crust is gravitational unstable and has higher density than undergoing mantle. It’s relatively clear that time of an extinction of the former volcanic front (ceasing of fluid and melt inflow to island arc crust) is ideal for delamination of main part of pyroxenites in the lower part of island arc crust. Sinking of delaminated blocks leads to magma generation due to both partial melting of the pyroxenites and disturb of the mantle. According this model the new source of the magma will form in the area of former volcanic front due to delamination and dehydratation of delaminated blocks of pyroxenites and amphibole pyroxenites of lower part of a island arc system. These magmas will be enriched in LREE, Nb, Ti, K in comparison with «tipical» island arc calc-alkaline magmas. Such geochemical signature is typical for subalkaline volcanic rocks, which erupts in former volcanic fronts. Sometimes heat flow could be enough for new act of upper crust melting after delamination and new magma influx. It could lead to silica-rich magmas appearance inside areas of subalkaline volcanism.
According suggested petrologic and geodynamic model of island arc evolution we can determine several stages: 1) primitive island arc with dominated fluid induced mantle melting; 2) developed island arc with combination of fluid-induced mantle melting and island arc crust melting; 3) mature island arc with former volcanic front and melting of delaminating blocks under this one. Assuming the model, the geochemical zoning of synchronous volcanism across mature (two or more chains) island arc system could be explained by several zones with different sets of magma sources due to evolution of island arc system and jumps of volcanic front position.
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