Abstract

Baddeleyite and zircon, including zircon overgrowths on baddeleyite, co-occur in a granophyric monzonite porphyry intruding volcanic rocks of the Majiahe Formation of the Xiong’er Group in the Xiaoshan area of the western Henan Province (China). It is inferred that the low silica content of the initial magma resulted in the formation of euhedral baddeleyite. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb dating of the baddeleyite yields a weighted mean 207Pb/206Pb age of 1779 ± 8 Ma, representing the crystallization of the porphyry. Survival of baddeleyite as the magma became silica saturated implies a rapid cooling process.

Zircon grains in the porphyry are anhedral to needle shaped, and are often clustered together within K-feldspar stringers or along the interface of quartz and K-feldspar in the granophyric groundmass, suggesting undercooling likely due to rapid emplacement. These observations indicate that zircon did not crystallize until final emplacement. Our U-Pb analyses of zircon yield a weighed mean 207Pb/206Pb age of 1777 ± 8 Ma, similar to that of baddeleyite. Subsequent medium- to high-temperature hydrothermal alteration affected most minerals at subsolidus conditions. Amphibole-plagioclase thermobarometry indicates pressure of 2.4–4.3 kbar and temperature of 470–580 °C for this alteration stage. Related alteration, such as total albitization of plagioclase, crystallization of secondary apatite, and sporadic secondary zircon overgrowths on baddeleyite, indicates that the fluid phase was enriched in Si, Na, and halogens (e.g., F, Cl). In addition to direct replacement of baddeleyite, zirconium required for the development of zircon overgrowths may also have been available through the alteration of Zr-bearing matrix phases such as amphibole and ilmenite.

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