Clinopyroxene megacrysts in Cenozoic volcanic rocks from Hainan Island and the Leizhou Peninsula, Southern China

Abstract

Three types of clinopyroxene megacrysts have been collected from Cenozoic basaltic rocks from Hainan Island and the Leizhou Peninsula in southern China. These megacrysts are dark green, black, and pale green in color, and are classified as Al-augite, Al-rich Fe-Na salite, and endiopside, respectively. Although they have different geochemical compositions, they all exhibit convex-upward, MREE-enriched patterns. The Al-augite megacrysts contain 8.45 to 9.01% Al2O3, 1.26 to 1.59% TiO2, 1.60 to 1.88% Na2O; have Mg# values of 69.5 to 75.4; and possess ΣREE contents of 24.7 to 26.9 ppm. Their LREE contents are enriched relative to the HREE (La/Ybn = 3.05 to 4.09). The Al-rich Fe-Na salite megacrysts have higher FeOT (10.4 to 11.4%) and Na2O (2.07 to 2.78%) contents, and lower Mg′ values (54.9 to 59.9), than do the Al-augite megacrysts. They contain high ΣREE contents (29.4 to 41.9 ppm) and have pronounced LREE/HREE enrichment (La/Ybn, = 3.86 to 20.6). Their Al2O3 contents (10.1 to 12.2%) are much higher than those of the Al-augite megacrysts (8.45 to 9.01%). One Al-rich Fe-Na salite megacryst has 87Sr/86Sr of 0.702913 ± 25 and 143Nd/144Nd of 0.512939 ± 20. The endiopside megacrysts possess high En and low Fs contents (Mg# values up to 89.3), and are characterized by high Cr2O3 (0.35 to 0.61%) and low Al2O3 (6.38 to 6.98%),TiO2 (0.30 to 0.48%), and ΣREE (7.71 ppm) contents. They have similar values of normalized LREE and HREE (La/Ybn = 1.09). One endiopside megacryst has 87Sr/86Sr of 0.703946 ± 40 and 143Nd/144Nd of 0.512816 ± 18. The Al-rich Fe-Na salite megacrysts in this study can be distinguished compositionally from Fe-Na salite megacrysts of previous studies. On the basis of the present study, these megacrysts are considered representative of a distinctive type of megacryst that may have a unique origin. Prior to this, endiopside megacrysts have not been described from alkaline basalts. A total of four types of clinopyroxene megacrysts now can be described from Cenozoic basaltic rocks worldwide - i.e. Al-augite, Fe-Na salite, Al-rich Fe-Na salite, and endiopside.