Basalt from the Extinct Spreading Center in the West Philippine Basin: New Geochemical Results and Their Petrologic and Tectonic Implications

Zhengxin Yin,Zhengyuan Li,Qiang Liu,Weiping Wang,Anyuan Xie,Liang Chen

doi:10.3390/min11111277

Abstract

We present geological, bulk-rock geochemical and Sr–Nd–Hf isotopic data for mafic rocks from the West Philippine Basin (WPB). These mafic rocks comprise pillow basalts characterized by a vesicular structure. The mid-ocean ridge basalt (MORB)-normalized trace element patterns of basalts from the study area display depletions in Nb. In addition, the chondrite-normalized lanthanide patterns of basalts from the WPB are characterized by significant depletions in the light lanthanides and nearly flat Eu to Lu segments. The investigated rocks have initial 87Sr/86Sr ratios (87Sr/86Sr(i)) of 0.703339–0.703455 and high εNd(t) values (8.0 to 8.7). Furthermore, basalts from the WPB have 176Hf/177Hf ratios that range from 0.28318 to 0.28321 and high εHf(t) from 15.2 to 16.3. Semi-quantitative modeling demonstrates that the parental melts of basalts from the study area were derived by ~20% adiabatic decompression melting of a rising spinel-bearing peridotite source. The Sr–Nd–Hf isotopic compositions of basalts from the WPB indicate that their parental magmas were derived from an upper mantle reservoir possessing the so-called Indian-type isotopic anomaly. Interpretation of the isotopic data suggests that the inferred mantle source was most likely influenced by minor inputs of a sediment melt derived from a downgoing lithospheric slab. Collectively, the petrographic and geochemical characteristics of basalts from the study area are analogous to those of mafic rocks with a back-arc basin (BAB)-like affinity. As such, the petrogenesis of basalts from the WPB can be linked to upwelling of an Indian-type mantle source due to lithospheric slab subduction that was followed by back-arc spreading.

Highlights

Introduction and Rationale of the StudyThe Philippine Sea Plate (PSP) is a significant element of the large-scale lithospheric organization of our planet that has been affecting the geodynamic evolution of the Western Pacific area since the Mesozoic eon
An important compositional aspect of the mafic rocks that have been recovered from the PSP is that they show a wide range of geochemical affinities, varying from normal mid-ocean ridge basalts (N-MORB) to enriched MORB (E-MORB) and oceanic island basalts (OIB) [1,2,3,4,5,6,7,8,9,10,11,12]
This indicates that the parental melt of basalts from the West Philippine Basin (WPB) were generated at much shallower depths than those of typical plume-derived basalts [43]

Summary

Introduction and Rationale of the Study

The Philippine Sea Plate (PSP) is a significant element of the large-scale lithospheric organization of our planet that has been affecting the geodynamic evolution of the Western Pacific area since the Mesozoic eon. The third hypothesis views the development of the WPB as influenced by both back-arc spreading and a mantle plume [10,11,12,16,17] All these models highlight the need for more precise data on the petrologic nature of basalts from the WPB to determine the geological origin of this specific basin of the PSP. A detailed report is provided on the petrographic, geochemical, and Sr–Nd–Hf isotopic data for the mafic rocks we recovered from the WPB

Geological Setting

Discussion

Source of Basalts from the WPB and Melting Conditions

Insights into the Origin of Basalts from the WPB

Conclusions