Geochemistry and Nd, Sr, Pb isotopic characteristics of the alkali-rich intrusive rocks in Alxa Fault Block, Western Inner Mongolia and their implications.

Abstract

The alkali-rich intrusive rocks in Alxa Fault Block are characterized by (1) supersaturation of silica, (with appearance of quartz and concentrations of SiO_2 reaching 62.01%~67.28%), (2) meta-aluminum((wt% Al_2O_3=)12.20%~16.83%), (3) alkali-rich (wt% K_2O+Na_2O=11.14%~14.18% and the Rittmann Index (σ) varing from 6.40 to 8.65), (4) potassium-rich (K_2O/Na_2O=1.01~1.79) and (5) low concentrations of Fe and Mg. The total rare earth element concentrations (∑REE) of the rocks are relatively high, with the REE patterns being sub parallel and with strong LREE enrichment. Multi-element spidergrams exhibit strong LILE and LREE enrichments and Nb, Ta, Ti, P depletion . The (above) together with low Nd (e(Nd,t)=-7.3~-9.4) and high Sr (e(Sr,t) =(+53.9~+99.9)) indicate that these rocks originated from an enriched upper mantle but were contaminated by crustally derived materials. Nd, Sr and Pb plots show that the alkali-rich intrusive rocks in Alxa have common features with Indosinian alkali-rich intrusives in Yanliao-Yinshan area. However, they are different from alkali-rich intrusive rocks from the north margin of the Tarim Block.The spatial and temporal distribution data, the isotopic characteristics and the geodynamic settings support the conclusion that the Alxa alkaline-rich intrusive rocks are the westward extension of the Indosinian alkaline-rich intrusion belt of the northern margin in North China Block. The Alxa Fault Block and the North China Block at least since the Late Paleozoic to Indosinian period.