Geochemical characteristics of apatite in metabasic rocks under different metamorphic conditions: a case study from the Paleoproterozoic Trans-North China Orogen

Abstract

Objective Apatite is a common accessory mineral that is widely distributed in various rock types. Its U-Pb age, trace elements (particularly REE, Th, U, and Sr), and Sr-Nd isotopic compositions provide important information on its chronology and magmatism. However, the geochemical behavior at different metamorphic levels during orogenesis remains unclear. As a typical continent-to-continent collisional orogenic belt in the Paleoproterozoic, the Trans-North China Orogen (TNCO) has recorded an integrated metamorphic sequence ranging from greenschist to amphibolite to granulite facies. Therefore, it is an ideal area to study the geochemical behavior of apatite during various grades of metamorphism involving the orogenic process. Methods In this study, we systematically collected metabasic samples of different metamorphic grades, including greenschist, amphibolite, and mafic granulite, in the Wutai-Hengshan area of the TNCO. We conducted detailed petrographic observations and geochemical analyses of apatite grains from metabasic rocks with different metamorphic grades. Results Our results showed that the apatite grains from the greenschist samples had both magmatic and metamorphic origins. The apatite grains in the amphibolite samples were mainly of metamorphic origin. In contrast, the grains from the granulite samples were closely related to crustal anatexis, exhibiting geochemical characteristics of magmatic-origin apatite. Conclusion This study shows that trace element variations in apatite can clearly reflect the influence of metamorphic grades, crustal anatexis, and coexisting rock-forming minerals with variations in temperature and pressure conditions during metamorphism. Significance The results of this study provide new constraints to our understanding of elemental migration and the geochemical balance within apatite during orogeny.

Keywords

• north china craton
• trans-north china orogen
• metabasic rocks
• metamorphism
• apatite
• trace elements