Occurrence and composition of sphene from eastern fold belt, Mount Isa Inlier, Cloncurry, northwestern Queensland, Australia

Abstract

Sphene is very common in rocks including albitized granite, dioritic porphyrite, calcsilicate rock and breccia from the eastern fold belt of Mount Isa Inlier, Cloncurry. Two stages of sphenes are present in these rocks. First-staged sphene is relatively fine, euhedral, some grains show round or patchy zoning; second-staged sphene is relatively large, anhedral to subhedral, some grains show patchy zoning; both possibly contain rutile, ilmenite and magnetite inclusions. All sphenes are of low-Al type. The second-staged sphene has lesser Fe apfu than the first-staged sphene. Light-color part of the sphene has bigger Fe apfu than the dark-color part, as observed on one individual grain of sphene with patchy zoning, the average XFe of the sphene with patchy zoning is greater than that of the sphene without patchy zoning. Because the sphenes are taken from different types of rocks, Si, Ti, and Al have variable relations with F + OH apfu. Si and Ti are not correlated with OH + F in all analyzed samples; Fe is correlated with OH + F in the sphene just from granite and dioritic porphyrite; Al is correlated with OH + F in the sphene from granite and breccia and is not correlated with OH + F in the sphene from the dioritic prophyrite and calcsilicate rocks. The first-staged sphenes were possibly formed in the processes of magmatism and metamorphism. The second-staged sphenes were formed as a result of the breakdown of hornblendes and biotites in the process of Na (Ca)-metasomatism.