Apatite in carbonatite preserving primary magmatic, post-magmatic overgrowth, and post India-Asia collision-related hydrothermal alteration: evidenced from U–Pb isotope and trace element geochemistry

Rashid, Mehboob ur; Rehman, Hafiz; Lee, Hao-Yang ; Sun-Lin, Chung; Das, Kaushik; Aoki, Kazumasa; Zeb, Muhammad Jawad; Ahmad, Nasir; Hussain, Muhammad; Yamamoto, Hiroshi

U–Pb isotope and trace element geochemistry of apatite combined with whole-­ rock geochemistry from four carbonatite bodies
(Sillai Patti “SP”, Loe Shilman “LS”, Warsak “WC”, and Jambil “JC”) in the Peshawar Plain Alkaline Igneous Province (PPAIP),
western Himalaya were used to elucidate petrogenetic source, magma evolution, emplacement timing, and post-­ magmatic hy-
drothermal/metasomatism of carbonatites. Whole-­ rock geochemistry classifies the studied bodies as calcio-­ carbonatites, except
WC, which shows a transition from calcio-­ to ferruginous variety. Major/trace element geochemistry suggests primary magmatic
records in SP and LS samples, localized fluid-­ assisted chemical modifications in WC samples, and intense metasomatism in JC
samples. Three types of apatites were identified in the four carbonatite bodies. Well-­ developed, euhedral, concentric zoned apa-
tite grains, exhibited by CL-­ dark homogeneous cores in SP, LS, and WC, are termed as Group I. Apatite grains with irregular or
spongy-­ type internal structures, spotted CL-­ bright patches or overgrowths in LS, SP, and WC are classified as Group II. Whereas
structurally and geochemically distinct, possessing murky textures and unclear growth domains in JC were classified as Group
III. The U–Pb isotope ratios from the CL-­ dark inner and homogeneous domains of Group I apatite in WC and SP carbonatites
yielded concordia ages of 280 and 268 Ma, respectively, indicating primary magmatic stage of the carbonatite emplacement,
whereas 199 Ma is from apatite from LS samples. The CL-­ bright outer domains of Group II apatite in LS, SP, and WC showed ages
of 101, 96, and 91 Ma, respectively, suggesting overgrowth or recrystallization due to an additional pulse of magmatism. Group III
apatite in JC gave U–Pb age values of 29 Ma, showing a complete overprint on the earlier magmatic records. Trace and rare-­ earth
element data in Group I apatite reveal carbonated mantle-­ derived melts, slightly depleted REE values with some chemical modi-
fication in Group II apatite linked with recrystallization or overgrowth, and highly depleted REE and chemically distinct Group
III apatite indicating fluid-­ mediated metasomatism. Apatite textural features, trace-­ element geochemistry, and geochronological
records indicate differing magma fertility and REE enrichment that was potentially linked with progressive magmatic evolution and late-­ stage fluid activity. The newly obtained U–Pb isotope and geochemical data from apatite and geochemical signatures in
whole rock provide strong evidence elucidating the protolith information of PPAIP carbonatites and late-­ stage metasomatism.

Geological Journal