CRYSTAL CHEMISTRY OF THE CONSTITUENT PHASES OF ULTRAMAFIC XENOLITHS FROM HANNUOBA REGION (NE CHINA)

Abstract

Because of the general inaccesibility of the lithospheric mantle, deep-seated xenoliths represent an important source of information on the earth’s interior. They are usually entrained by alkaline basalts in circumcratonic belts or by kimberlites and/or lamproites in cratonic areas. In the last decades the crystal chemistry of major mantle constituents has improved the knowledge of mantle mineralogy and has emerged as an additional petrological constraint for evaluating the cooling histories of the host rocks (Dal Negro et al., 1982; Saxena and Dal Negro, 1983; Cundari et al., 1986; Princivalle et al., 2000). In this work the crystal chemistry of coexisting phases in a suite of spinel peridotite xenoliths from Hannuoba region has been carried out by means of single crystal X-ray diffraction and electron microprobe to obtain site occupancies. The Hannuoba basaltic plateau, 200 km northwest of Beijing, large more than 1700 km2, lies within the North China Craton and consists of interlayered alkaline and tholeiitic basalts; the alkaline ones host deep-seated xenoliths while only very rare xenoliths have been found in the tholeiites. Volcanism is related to widespread Cenozoic rifting in the North China Craton. Along the southern margin of the Hannuoba plateau there are about forty xenolith-rich zones; in particular one of those, Damaping, is noteworthy for its abundant spinel peridotite xenoliths (approximately 85% of the outcrop), their large size (up to 80 * 60 * 45 cm), their coarse grain size and gem-quality olivine. The “peridotitic body” outcrops along a NE-trending tectonic belt. All the xenoliths investigated in this study were collected at Damaping. X-ray diffraction data were recorded on an automated KUMA-KM4 (K-geometry) diffractometer using MoK_ radiation, monochromatised by a flat graphite crystal. Data collection and structure refinement strategies were performed following procedures used for cpx by Dal Negro et al. (1982), for spinel by Della Giusta et al. (1986) and for olivine by Princivalle and Secco (1985). The same crystals used for X-ray single crystal data collection were also used for chemical analyses. These were performed by a CAMECA- CAMEBAX microprobe operating at 15 KV and 15 nA. A PAP-CAMECA program were used to convert X-ray counts into weight percent of the corresponding oxides. Results are considered accurate to within 2-3 % for major and better than 10 % for minor elements. Clinopyroxenes The Hannuoba clinopyroxenes (cpx), characterised by high Ca (0.775-0.872 atoms for formula unit, a.f.u.) and Mg (0.791-0.962 a.f.u.) contents, plot in the aluminous-diopside field (Burnham et al., 1967). Within the suite Vcell (cell volume) varies from 433.34 to 436.44 and parallels VM1 and VM2 (M1 and M2 site volume, respectively) increase. On the contrary, T site volume decreases with increasing Vcell, due to increasing Si content and this is related to the increase of mg# cpx and the general decrease of cpx modal content (i.e. increase of basalt component extraction). As is typical for pyroxenes from ultramafic inclusions T site is near full occupied by Si4+ (1.878-1.945 a.f.u.). M2 site is essentially filled by Ca and Na, with minor amounts of Mg and Fe2+. Substantial occupancy of this site by relatively large ions like Ca and Na tends to minimize the distortion of M2 polyhedron (Dal Negro et al., 1982). Mg and R3+ (AlVI+Fe3++Cr+Ti) cations fill M1 polyhedron, while Fe2+ is present only in small amount. M1 volume increase is controlled by R3+ decrease. Spinel Within the suite, unit cell parameter ao ranges from 8.1291 to 8.2265 A and Cr content increases from 0.1792 to 0.6191 a.f.u., whereas oxygen coordinate u remains virtually constant (0.26276-0.26310), therefore giving a quite constant ratio of octhahedral (M-O) and tetrahedral (T-O) bond distances. The increase in Cr causes progressive ordering of Mg and Al in T and M sites respectively. Princivalle et al., (1989) evidenced how the cation partitioning and the consequently oxygen parameter (u) of Cr-spinels from peridotites is related to the cooling history of the rock. Hannuoba spinels are characterised by higher u values (compared with analogues spinels described in literature), suggesting a slower cooling history. Olivine Hannuoba investigated olivines have forsteritic compositions (Fo 89.2-90.6 %) and Vcell varing from 291.6 to 291.8 A3. VM1 (11.876-11.907 A3) increases with decreasing mg#, due to larger ionic radius of Fe2+ relative to that of Mg, according to Shannon’s (1976) data. Mean M1-O and M2-O bond lengths increase slightly as Fo content decrease. Fe2+ and Mg entry in both the octahedral sites M1 and M2. No significant ordering of Fe2+ on M1 site has been detected (KD values near the unit).