Abstract
A series of Neoproterozoic boninitic pillow lavas are hosted in Early Neoproterozoic strata along the southeastern margin of the Yangtze Block. These lavas record a period of subduction initiation during the secular evolution of South China. In this work we present analytical results from three units of pillow lavas (the Zhangyuan, Shexian and Lushan units) that form a common liquid line of descent with the previously reported 830-Ma boninites in the region, suggesting that they were produced by differentiation from similar parental melts. MELTS calculations show that major element compositions require fractionation of olivine+clinopyroxene+plagioclase+spinel from primary boninitic melts to have been accompanied by variable amounts of crustal contamination. The pillow lavas from the three localities show similar rare earth element patterns and arc-like primitive mantle normalized trace element patterns. They have high 206Pb/204Pb (18.41–19.33), 207Pb/204Pb (15.63–15.79) and 208Pb/204Pb ratios (38.78–40.26). Their εNd values decrease from the Lushan lavas (+0.7 to +1.9) to the Shexian lavas (−0.5 to −1.1) to the Zhangyuan lavas (−4.0 to −1.0), reaching values lower than the upper-level crustal contaminants and therefore indicating progressive enrichment of their sources by sediment melts. Compared with nearby but older (850Ma) MORB-like tholeiitic basalts, which yield mantle potential temperature below 1400°C and H2O contents nearly 0.13wt.%, the primary magmas of the boninite series rocks in this study were formed under higher pressures (3.5–1.0GPa) and mantle potential temperatures (1440–1500°C) with high H2O contents (3420 to 4830ppm), clear slab-derived sedimentary melt input, and harzburgite residues. The introduction of slab-derived water and sedimentary melt components into the mantle wedge at such high pressures, together with high temperatures and high degrees of melting, indicate that within a time span of only 20millionyears after subduction initiation (850 to 830Ma), the slab subducting under the southeastern margin of the Yangtze Block achieved substantial penetration to depth and interacted with hot overlying mantle, before subduction was abruptly ended by the amalgamation of the Yangtze and Cathaysia blocks.
Highlights
Boninites attracted much attention at the end of the 20th century, primarily, due to deep‐sea studies of forearc slopes in the modern plate convergence zones of the southwestern Pacific
Different types of the boninites have spectra of the same type with distinct negative anomalies of Nb(Ta) and Ti, positive anomalies of Sr and Zr(Hf) and apparent enrichment in large‐ionic lithophile (LIL) ele‐ ments (Rb, Ba, Cs, U, and Th) relative to N‐MORB. Such a kind of the trace elements patterns clearly suggests that (i) petrogenesis of boninite series should be ex‐ cluded any and even a minimum crustal input, and (ii) melting requires a mantle source more depleted in comparison to the mantle lherzolite that generates MORB melts
We reported a fragment of the suprasubduction ophiolites with sheeted dikes in gabbroids and me‐ talavas of the boninite series (~2.8 billion years) in the Iringora locality of the North‐Karelian greenstone belt [Shchipansky et al, 2001, 2004]
Summary
Boninites attracted much attention at the end of the 20th century, primarily, due to deep‐sea studies of forearc slopes in the modern plate convergence zones of the southwestern Pacific. De‐ tailed geochemical studies of many ophiolite sections revealed the presence of boninites unknown for mid‐ oceanic ridges, but abundant in juvenile island‐arc basements. As the boninite findings were ever‐growing in both the classi‐ cal fragments of the ‘ancient oceanic crust’, e.g. the Troodos in Cyprus, and Semail in Oman, or ‘classical sutures’ such as, the Main Urals Fault in Russia, this inevitably raises the question on the ophiolite nature. Another problem concerns the evolution of boninitic magmatism as boninites have been discovered in the early Precambrian greenstone belts. The aims of the paper are to provide an over‐ view of the boninitic magmatism occurrences through space and time, track its evolution, and propose a geo‐ dynamic explanation of this highly informative phe‐ nomenon
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
