High-Mg potassic rocks from Taiwan: implications for the genesis of orogenic potassic lavas

Abstract

Taiwan is an active mountain belt formed by oblique collision between the Luzon arc and the Asian continent. Regardless of the ongoing collision in central and southern Taiwan, a post-collisional extension regime has developed since the Plio -Pleistocene in the northern part of this orogen, and led to generation of the Northern Taiwan Volcanic Zone. Emplaced at 0.2 Ma in the southwest of the Volcanic Zone, lavas from the Tsaolingshan volcano are highly magnesian (MgO 15 wt.%) and potassic (K 2 O 5 wt.%; K 2 O/Na 2 O 1.6 -3.0). Whereas these basic rocks (SiO 2 48 wt.%) have relatively low Al 2 O 3 12 wt.%, total Fe 2 O 3 7.5 wt.% and CaO 7.2 wt.%, they are extremely enriched in large ion lithophile elements (LILE, e.g. Cs, Rb, Ba, Th and U). The Rb and Cs abundances, >1000 and 120 ppm, respectively, are among the highest known from terrestrial rocks. In addition, these rocks are enriched in light rare earth elements (LREE), depleted in high field strength elements (HFSE), and display a positive Pb spike in the primitive mantle-normalized variation diagram. Their REE distribution patterns mark with slight Eu negative anomalies (Eu/Eu * 0.90 -0.84), and Sr and Nd isotope ratios are uniform ( 143 Nd/ 144 Nd 0.51268 -0.51259). Olivine, the major phenocryst phase, shows high Fo contents (90.4 ± 1.8; 1r deviation), which are in agreement with the whole rock . Spinel inclusions in olivine are characterized by high Cr/Cr + Al ratios (0.94 -0.82) and have compositions similar to those from boninites that originate from highly refractory peridotites. Such petrochemical characteristics are comparable to the Group I ultrapotassic rocks defined by Foley et al. [Earth-Sci. Rev. 24 (1987) 81], such as orogenic lamproites from central Italy, Span and Tibet. We therefore suggest that the Tsaolingshan lavas resulted from a phlogopite-bearing harzburgitic source in the lithospheric mantle that underwent a recent metasomatism by the nearby Ryukyu subduction zone processes. The lavas exhibit unique incompatible trace element ratios, with Rb/Cs 8, Ba/Rb 1, Ce/Pb 2, Th/U 1 and Nb/ U 0.8, which are significantly lower than the continental crust values and those of most mantle-derived magmas. Nonmagmatic enrichment in the mantle source is therefore required. Based on published experimental data, two subductionrelated metasomatic components, i.e., slab-released hydrous fluid and subducted sediment, are proposed, and the former is considered to be more pervasive for causing the extraordinary trace element ratios observed. Our observations lend support to the notion that dehydration from subducting slabs at convergent margins, as a continuing process through geologic time, can account for the fractionation of these elemental pairs between the Earth's crust and mantle. D

Shoshonites in Southern Tibet Record Late Jurassic Rifting of a Tethyan Intraoceanic Island Arc

“…Shoshonites are not common in oceanic arcs and, where present, have been interpreted (Jakes and White 1972;Morrison 1980;Stern et al 1988;Gill and Whelan 1989) in various ways, including association with arc maturity, arc continent collision, arc seamount collision, and arc rifting as well as being indicators of depth to the Benioff zone. Of these, an arc continent collision such as that observed in Taiwan (Chung et al 2001) can be ruled out because geochemistry, sedimentology, and the age of the Zedong terrane preclude its development in this setting. The HFSE are noticeably more strongly depleted than those in shoshonitic rocks associated with arc continent collision.…”

SIMS U-Pb zircon dating and Re-Os isotopic analysis of the Hulu Cu-Ni deposit, eastern Tianshan, Central Asian Orogenic Belt, and its geological significance

“…Therefore, the Ce/Pb ratios (0.14-7.3) in most of these rocks, which are lower than in mantle-derived magmas (e.g. Ce/Pb ~ 25 for average oceanic basalts), may be due to the incorporation of Pb into the mantle via subduction fluids (Chung et al 2001). The low Pb isotopic ratios exclude the contribution from subducted sediment because the slight addition of sedimentary material would significantly alter Pb isotopic ratios toward more radiogenic values (De Hollanda et al 2003).…”

“…Marked enrichments in Pb may be ascribed to the presence of hydrous fluid because this element is soluble in aqueous slab-derived fluids (Seghedi et al 2004). Additionally, experimental data suggest that the Ce/Pb ratios of a slab-derived fluid would be as low as ~0.1 or even lower (Chung et al 2001), and the Ce/ Pb values less than 20 imply the influence of subductionrelated fluids within the asthenosphere (Seghedi et al 2004). The absence of a negative Ce anomaly indicates that these rocks were not affected by low-temperature alteration (Zou et al 2000).…”

Arc and Slab-Failure Magmatism in Cordilleran Batholiths II – The Cretaceous Peninsular Ranges Batholith of Southern and Baja California

“…To our knowledge, only one of the published petrogenetic models for these various igneous suites link them to possible slab failure (Whalen et al 2010), yet they are similar chemically to the La Posta suite, as well as Cenozoic examples, such as northernmost Taiwan, where comparable magmas were erupted as the exhumed mountain belt collapsed within a few m.y. of collision (Chung et al 2001;Wang et al 2002Wang et al , 2004 and Tibet, where enigmatic magmatism accompanied uplift and exhumation (Turner et al 1993(Turner et al , 1996Mahéo et al 2002;Kohn and Parkinson 2002). These two examples of probable slab failure magmatism might typify the shallow and deep break-off end members respectively, as reflected in their orogenic width, proximity to the suture, and differences in timing relative to the collision.…”