Sensitive High-Resolution Ion Microprobe U-Pb Dating of Baddeleyite and Zircon from a Monzonite Porphyry in the Xiaoshan

Research Paper

GEOSPHERE Sensitive high-resolution ion microprobe U-Pb dating of baddeleyite and zircon from a monzonite porphyry in the Xiaoshan

GEOSPHERE; v. 12, no. 4 area, western Henan Province, China: Constraints on baddeleyite doi:10.1130/GES01328.1 and zircon formation process 9 figures; 3 tables Linlin Li1, Yuruo Shi1, J. Lawford Anderson2, and Minli Cui3 1Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Baiwanzhuang Road 26, Xicheng, Beijing 100037, China CORRESPONDENCE: shiyuruo@bjshrimp.cn 2Department of Earth and Environment, Boston University, 685 Commonwealth Avenue, Boston, Massachusetts 02215, USA 3Development and Research Center, China Geological Survey, Fuwai Street 45, Xicheng, Beijing 100037, China CITATION: Li, L., Shi, Y., Anderson, J.L., and Cui, M., 2016, Sensitive high-resolution ion microprobe U-Pb dating of baddeleyite and zircon from a monzo ABSTRACT and LeCheminant, 1993; Scoates and Chamberlain, 1995; Klemme and Meyer, nite porphyry in the Xiaoshan area, western Henan 2003; Rajesh and Arai, 2006; Xiang et al., 2012). Baddeleyite has relatively high Province, China: Constraints on baddeleyite and Baddeleyite and zircon, including zircon overgrowths on baddeleyite, concentrations of U, negligible common Pb, and is highly resistant to Pb loss zircon formation process: Geosphere, v. 12, no. 4, p. 1362–1377, doi:10.1130/GES01328.1. co-occur in a granophyric monzonite porphyry intruding volcanic rocks of the because it has a high closure temperature for the diffusion of Pb (Wingate Majiahe Formation of the Xiong’er Group in the Xiaoshan area of the western and Compston, 2000). In addition, baddeleyite is typically magmatic in origin Received 18 February 2016 Henan Province (China). It is inferred that the low silica content of the initial and does not commonly occur as xenocrysts (Wingate and Compston, 2000), Revision received 25 May 2016 magma resulted in the formation of euhedral baddeleyite. Sensitive high-reso and thus baddeleyite geochronology can be used to constrain the timing of Accepted 24 June 2016 lution ion microprobe (SHRIMP) U-Pb dating of the baddeleyite yields a igneous crystallization. For example, baddeleyite has been extensively applied Published online 12 July 2016 weighted mean 207Pb/206Pb age of 1779 ± 8 Ma, representing the crystallization for dating of alkaline syenites (Heaman and Machado, 1992; Teixeira et al., of the porphyry. Survival of baddeleyite as the magma became silica saturated 1997), carbonatites (Reischmann et al., 1995), and mafic igneous rocks such as implies a rapid cooling process. kimberlites, layered mafic intrusions, gabbro, and diabase sills (Krogh et al., Zircon grains in the porphyry are anhedral to needle shaped, and are often 1987; Heaman and Grotzinger, 1992; Heaman and LeCheminant, 1993; Schärer clustered together within K-feldspar stringers or along the interface of quartz et al., 1997; Kerschhofer et al., 2000; Wingate and Compston, 2000; Li et al., and K-feldspar in the granophyric groundmass, suggesting undercooling likely 2006; Li et al., 2013). In contrast, baddeleyite in intermediate igneous rocks is due to rapid emplacement. These observations indicate that zircon did not rarely reported. crystallize until final emplacement. Our U-Pb analyses of zircon yield a weighed Polycrystalline zircon aggregates often develop as rims on preexisting mean 207Pb/206Pb age of 1777 ± 8 Ma, similar to that of baddeleyite. Subsequent baddeleyite during high-grade metamorphism (Davidson and van Breemen, medium- to high-temperature hydrothermal alteration affected most minerals 1988; Patterson and Heaman, 1991; Heaman and LeCheminant, 1993; Wingate at subsolidus conditions. Amphibole-plagioclase thermobarometry indicates et al., 1998; Lumpkin, 1999; Rioux et al., 2010). Zircon overgrowths can also pressure of 2.4–4.3 kbar and temperature of 470–580 °C for this alteration stage. be formed during hydrothermal alteration (Heaman and Grotzinger, 1992; Related alteration, such as total albitization of plagioclase, crystallization of Heaman and LeCheminant, 1993; Wingate, 2001). In some cases, baddeleyite secondary apatite, and sporadic secondary zircon overgrowths on baddeleyite, may be completely replaced by polycrystalline zircon aggregates (Davidson indicates that the fluid phase was enriched in Si, Na, and halogens (e.g., F, Cl). and van Breemen, 1988). As a high field strength element (HFSE), zirconium In addition to direct replacement of baddeleyite, zirconium required for the de- (Zr) is usually stable in host minerals, but when fluid is present, it can beeasily velopment of zircon overgrowths may also have been available through the transported as halogen or hydroxyl complexes (Rasmussen, 2005; Migdi alteration of Zr-bearing matrix phases such as amphibole and ilmenite. sov et al., 2011; Ayers et al., 2012; Bernini et al., 2013). It is noteworthy that baddeleyite and zircon are not the sole Zr-rich phases. Hornblende and ilmen- INTRODUCTION ite are also the major carriers of Zr (Rubin et al., 1993; Fraser et al., 1997; Bingen et al., 2001; Charlier et al., 2007). U-Pb geochronology has been improved dramatically through the recent In this study coexisting baddeleyite and zircon grains as well as zircon development of in situ techniques. Zircon is common in silica-saturated rocks, overgrowths on baddeleyite were found in a monzonite porphyry intruding

For permission to copy, contact Copyright whereas the mineral baddeleyite (ZrO2) occurs primarily as a U-bearing acces- volcanic rocks of the Majiahe Formation of the Xiong’er Group in the Xiaoshan Permissions, GSA, or [email protected]. sory phase in igneous rocks with low silica content (Kamo et al., 1989; Heaman area (western Henan Province, China). The monzonite porphyry shows grano-

phyric texture and underwent wide and intense hydrothermal alteration, which plagioclase phenocrysts (Figs. 2C and 3A). Hornblende is usually replaced by also influenced the Xiong’er Group widely in the Waifangshan, Xiong’ershan, chlorite and biotite from the core to rim. Widely distributed ilmenite grains oc- and Xiaoshan areas (Zhao et al., 2001; Han et al., 2006). We present petrog- cur closely associated with hornblende (Figs. 2D–2F and 3B–3F) and are inten- raphy, amphibole-plagioclase thermobarometry, and sensitive high-resolution sively altered to titanite along crystal boundaries and fractures (Figs. 3B–3F). ion microprobe (SHRIMP) U-Pb geochronology of baddeleyite and zircon to Apatite usually occurs at the margin of altered hornblende (Figs. 2D, 3B, and better evaluate the processes of baddeleyite and zircon crystallization and zir- 3D) and also shows close spatial relationship with secondary titanite (Figs. con overgrowth on baddeleyite. 3B–3E); some grains cut through hornblende, ilmenite, and the granophyric groundmass (Figs. 3E, 3F). K-feldspar is also often altered by kaolinization and GEOLOGICAL SETTING AND SAMPLE DESCRIPTIONS sericitization. The granophyric groundmass is composed of intergrowths of K-feldspar The Xiong’er volcanic belt occurs at the southern margin of the North and quartz on a 1 mm scale and occurs around euhedral feldspar phenocrysts. China craton covering an area of >60,000 km2 (Fig. 1A; Zhao et al., 2002). The K-feldspar and quartz intergrowths coarsen as they emanate from phenocrysts belt is bound to the northwest by the Jiangxian-Lintong fault, to the north- and form fan-like splays. Quartz distributed in K-feldspar varies in habit from east by the Luoyang-Baofeng fault, and to the south by the Luonan-Luanchuan cuneiform to vermicular and has a parallel to plumose and radiating texture fault, which separates it from the North Qinling orogenic belt (Fig. 1B; Zhao (Figs. 2F, 2G, 3G, and 3H). Some quartz grains are round or triangular (Figs. 2H et al., 2009). The Xiong’er volcanic rocks are mainly distributed in the Zhong- and 3I), indicative of the b-quartz polymorph. Intergrown K-feldspar and quartz tiaoshan, Xiaoshan, Xiong’ershan, and Waifangshan areas. The rocks uncon- in the granophyric groundmass often have the same extinction direction, indi- formably overlie Archean to Paleoproterozoic crystalline basement, and are cating concurrent growth. unconformably overlain by Mesoproterozoic–Neoproterozoic terrigenous sandstones, limestones, and calc-silicate rocks (Zhao et al., 2009). The Xiong’er ZIRCONIUM-RICH ACCESSORY MINERALS Group has been divided, from bottom to top, into the Dagushi, Xushan, Jidan- ping, and Majiahe Formations, which are composed of basaltic andesite, ande We conducted scanning electron microscope analyses on these samples site, dacite, and rhyolite pyroclastic units interlayered with minor sedimentary in order to better understand the occurrence of the Zr-rich accessory min-

interbeds (Zhao et al., 2002). Previous age data for the volcanic rocks based on eral phases including baddeleyite, zircon, and minor zirconolite (CaZrTi2O7). K-Ar, Rb-Sr analyses of minerals or whole-rock samples, conventional multi Baddeleyite occurs as a euhedral and platy mineral and also as inclusions in grain U-Pb zircon analyses, and several Sm-Nd analyses (Qiao et al., 1985; hornblende, plagioclase, and K-feldspar and along their boundaries (Figs. 4A– Zhang et al., 1994; Ren and Li, 1996) indicated that these rocks formed from 4C). Zircon occurs partially as isolated crystals and as sporadic rims (<5 mm) on 1700 to 1400 Ma (Bai, 1993; Ren et al., 2000 and references therein). However, baddeleyite. Isolated zircon grains are anhedral and small (<100 mm), and zir- such ages are considered unreliable because of large errors and mobilization cons with irregular or acicular shape tend to cluster together and occur contin- of K-Ar and Rb-Sr during later alteration and mixing of different components. uously along the secondary chlorite (Fig. 4D) located in the boundary of plagio In recent years, high-precision geochronology of Xiong’er volcanic rocks, clase and quartz or distributed along K-feldspar stringers in the granophyric using SHRIMP and laser ablation–inductively coupled plasma–mass spectrom- groundmass (Figs. 4E, 4F). Needle-shaped zircon grains are observed growing etry (LA-ICP-MS) analyses, indicates that most of the volcanic rocks formed at along the interface of intergrown K-feldspar and quartz (Fig. 4G). Zircon over- 1.80–1.75 Ga (Zhao et al., 2004; Peng et al., 2008; He et al., 2009). Coeval mafic growths are generally small, sporadic, and occur mostly in the fractures and to felsic subvolcanic rocks intruded the Xiong’er volcanic rocks and the meta- margins of baddeleyite (Fig. 4H). Only a few baddeleyite grains included in morphic basement in this area (Pang and Yan, 2004). altered hornblende have continuous zircon rims (Figs. 4I, 4J). Zirconolite is The monzonite porphyry of this study intruded the Majiahe Formation and distributed in plagioclase and quartz and was replaced by titanite along their is locally covered by Cenozoic sediments in Duoyang area, the southeastern margins (Figs. 4K, 4L). part of Xiaoshan area (Fig. 1C). Three drill-core samples of the porphyry at different depths were collected (long 111°22′45.9″E, lat 34°24′47.4″N). BADDELEYITE AND ZIRCON U-Pb DATING The monzonite porphyry samples are granophyric (Figs. 2A, 2B) and have phenocrysts consisting of plagioclase (30%), hornblende (22%), K-feldspar Analytical Methods (13%), and biotite (5%). The groundmass is dominated by granophyric inter- growth of K-feldspar (15%) and quartz (12%). The accessory minerals include Baddeleyite and zircon were dated on the SHRIMP II at the Beijing SHRIMP ilmenite (3%), baddeleyite, zircon, apatite, rutile, and zirconolite. Center, Institute of Geology, Chinese Academy of Geological Sciences. The Plagioclase is euhedral and tabular, and has undergone epidotization. analytical procedures were similar to those of Williams (1998) and data were Thin quartz veins containing fine-grained epidote cut through the cleavage of processed using the Excel-based Squid and Isoplot programs (Ludwig, 2001).

GEOSPHERE | Volume 12 | Number 4 Li et al. | Geochronology and formation process of coexisting baddeleyite and zircon Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1362/3337296/1362.pdf 1363 by guest on 25 September 2021 on 25 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/12/4/1362/3337296/1362.pdf Research Paper J 3 iacu 3 B A 4°N 5°N C 0 0 10 9°E n Archean-Paleoproterozoi Meso-Neoproterozoi c cover s Mts. 0 Meso-Neoproterozoi c sedimentar Archean-Paleoproterozoi Xiong’e r volcanic Kuanping Qinlin g Complex N N a Basaltic P Sedimen Cenozoi Andesite Proterozoi c porphyry Monzonite Proterozoi 300 ndesite roterozoi Q 10 0 km ilia L km ugou n Comple

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GEOSPHERE | Volume 12 | Number 4 Li et al. | Geochronology and formation process of coexisting baddeleyite and zircon 1364 Research Paper

A B

C D Pl Pl Ep Ilm Hbl Figure 2. (A, B) Drilling hole samples. Pl (C) Several fine-grained epidotes (Ep) distributed along a thin quartz vein cutting Ap through cleavage of plagioclase pheno 200 m 5 cryst. (D) Needle-like apatite (Ap) grains ˩ 00 ˩m distributed in the margin of hornblende (Hbl). (E) Ilmenite (Ilm) occurring closely associated with hornblende. (F, G) The E F granophyric intergrowths of K-feldspar Hbl Hbl (Kfs) and quartz (Qtz) around euhedral plagioclase (Pl) phenocrysts. (H) Quartz grains distributed in K-feldspar showing Kfs+Qtz Ilm round and triangular shape. Ilm Kfs+Qtz Pl 500 ˩m 500 ˩m G H

Kfs Kfs+Qtz

Pl Qtz 500 ˩m 500 ˩m

A B C Hbl Ilm Hbl Ep Ap Ap Hbl Ilm Ttn Ilm Ilm Ap Pl Pl Ttn Ttn Ap Pl Ap Hbl Qtz m m Kfs m Figure 3. Backscattered electron images ˩ ˩ ˩ showing petrographic characteristics of the rock samples. (A) Fine-grained epidote (Ep) distributed along with a thin quartz D E F (Qtz) vein cutting through cleavage of Pl Ttn Ilm Hbl Ilm plagioclase (Pl) phenocryst. (B–F) Ilmenite Ap Ilm (Ilm) occurring with hornblende (Hbl) and replaced by titanite (Ttn) in the margin and Ap Ttn Ap Hbl cracks; apatite (Ap) crystals are scattered in Hbl Hbl Kfs Ap the margin of altered hornblende and show a close spatial relationship with secondary Ap Ap Ilm Ilm Ilm titanite (B–E); some grains cut through Ap Qtz hornblende, ilmenite, and the granophyric Ttn Kfs groundmass (E, F). Kfs—potassium feld- m ˩m ˩m spar. (G, H) The granophyric intergrowths ˩ of K-feldspar and quartz around euhedral feldspar phenocrysts. (I) Round and trian- G H I gular quartz grains distributed in K-feldspar. Hbl

Kfs+Qtz Kfs Kfs+Qtz Pl Pl Kfs+Qtz ˩m ˩m Hbl ˩m

Mass resolution during the analytical sessions was ~5000 (1% definition). Spot eliminate the errors induced by the electrical conductivity difference between sizes were 25–30 mm and each site was rastered for 180 s prior to analysis to re- the test target and the unknown sample target. Common lead corrections were 90 16 + 204 + 206 + 207 + 204 move the gold coating. Five scans on Zr2 O , Pb , background, Pb , Pb , made using the measured Pb. The errors for individual analyses in the data 208Pb+, 238U+, 232Th16O+, and 238U16O+ were made for both baddeleyite and zircon. table and figures are quoted at s1 , whereas errors for weighted mean ages are Standard zircon M257, with an age of 561.3 Ma and U content of 840 ppm (Nas- quoted at 2s. dala et al., 2008), was used to calibrate U and Th content. For baddeleyite and zircon dating, Pb+/U+ ratios were calibrated with the power law relationship between Pb+/U+ and UO+/U+ relative to the Phalaborwa baddeleyite standard and Dating Results the TEMORA zircon standard, dated as 2059.6 Ma (Heaman, 2009) and 417 Ma (Black et al., 2003), respectively. The standard M257 was conventionally an- The results of SHRIMP U-Pb analyses on baddeleyite and zircon from the alyzed prior to the unknown samples on the instrumental test target. In this monzonite porphyry sample are listed in Table 1. U-Pb analysis of baddeleyite, M257 was also pasted on the unknown target Baddeleyite occurs mostly as euhedral, platy, and monoclinic crystals as together with an unknown baddeleyite sample and Phalaborwa standard to much as 200 mm long with length:width ratios ranging from 1.2 to 2 (Fig. 5A).

A Pl B C Bad Kfs Bad Bad Bad Pl Pl Hbl ˩m ˩m ˩m D E F Pl Figure 4. Backscattered electron images Chl Qtz showing the occurrence of Zr-bearing ac- Kfs cessory mineral phases. (A–C) Euhedral, platy baddeleyite (Bad) crystals scattered Qtz in hornblende (Hbl), plagioclase (Pl), and Zrn boundary of plagioclase and K-feldspar. Zrn Qtz Kfs (D–F) Anhedral, fine-grained, isolated zir- Zrn con (Zrn) grains clustered together; zircon grains occur continuously along with sec- ˩m Pl ˩m ˩m ondary chlorite (Chl) located in the boundary of plagioclase and quartz (Qtz) (D) and distributed along with elongate K-feldspar G H I (Kfs) in the granophyric groundmass (E, F). (G) Needle-like zircon occurring along the Zrn Kfs Kfs Bad interface of intergrown K-feldspar and quartz. (H) Sporadic zircon overgrowths in Ttn Hbl Zrn the fractures and margin of baddeleyite. Zrn (I, J) Continuous zircon rims on baddeleyite in altered hornblende. (K, L) Zirconolite (Zrl) distributed in quartz and plagioclase Qtz Bad Bad replaced by titanite in the margin. Zrn Pl Qtz ˩m ˩m ˩m

J K L Ttn Hbl Qtz Zrl Ttn Bad Zrl Pl Zrn Bad ˩m ˩m ˩m

TABLE 1. SHRIMP U-Pb ANALYTICAL RESULTS OF BADDELEYITE AND ZIRCON FROM THE MONZONITE PORPHYRY (LD03)

206 206 206 238 207 206 Pbc U Th Pb* ± ± ± Error Pb*/ U age Pb*/ Pb* age Discordant Spot (%) (ppm) (ppm) 232Th/238U (ppm) 207Pb*/206Pb* (%) 207Pb*/235U (%) 206Pb*/238U (%) corrected (Ma) (Ma) (%) LD03 Baddeleyites 1.1 0.05 175 9 0.05 44.8 0.10940.9 4.48 6.20.297 6.20.990 1678 ±911789±16 6.2 2.1 0.02 149 5 0.03 43.7 0.11040.8 5.19 6.20.341 6.20.991 1891 ±100 1806 ±15–4.7 3.1 0.02 241 12 0.05 65.2 0.1095 0.74.756.2 0.3146.2 0.9941762±95 1791 ±121.6 4.1 0.13 1154 0.04 29.5 0.10891.0 4.47 6.30.297 6.20.988 1678 ±911781±18 5.8 5.1 – 185 8 0.05 47.4 0.10920.8 4.50 6.20.299 6.20.992 1687 ±921785±14 5.5 6.1 0.06 150 9 0.06 37.3 0.1091 1.04.366.3 0.2906.2 0.9871641±90 1785 ±188.1 7.1 0.15 144 5 0.03 38.5 0.10830.9 4.63 6.30.310 6.20.989 1741 ±941771±17 1.7 8.1 0.10 123 5 0.04 31.5 0.10861.0 4.46 6.20.297 6.20.988 1679 ±911777±18 5.5 9.1 – 170 6 0.04 46.0 0.11041.0 4.81 6.30.316 6.20.988 1768 ±961807±18 2.1 10.1 – 101 5 0.05 27.3 0.10771.0 4.66 6.30.314 6.20.987 1759 ±951762±19 0.2 11.1 0.11 149 5 0.03 38.3 0.1069 0.94.426.3 0.3006.2 0.9901690±92 1747 ±173.2 12.1 0.23 230 9 0.04 62.4 0.1082 0.94.716.3 0.3166.2 0.9901769±96 1770 ±160.1 13.1 0.08 222 10 0.05 64.5 0.10830.7 5.05 6.20.338 6.20.994 1877 ±100 1771 ±13–6.0 14.1 0.11 123 4 0.04 35.2 0.10821.0 4.97 6.30.333 6.20.987 1853 ±100 1770 ±18–4.7 15.1 0.07 1194 0.04 33.1 0.1074 1.14.796.3 0.3246.2 0.9861807±98 1756 ±19–2.9 LD03 Zircons 1.1 0.36 610 409 0.69 147.0 0.10440.7 4.02 1.10.280 0.90.781 1589 ±131704±13 6.7 2.1 0.11 235 282 1.24 64.7 0.1075 0.94.741.3 0.3201.0 0.7461789±15 1758 ±16–1.8 3.1 0.29 449 374 0.86 112.0 0.1081 0.74.311.1 0.2890.9 0.8121637±13 1767 ±127.4 4.1 – 273 262 0.99 72.3 0.10920.6 4.64 1.10.308 0.90.843 1730 ±141787±11 3.2 5.1 0.25 297 170 0.59 69.1 0.1048 0.83.901.2 0.2700.9 0.7721540±13 1711 ±1410.0 6.1 0.91 732 595 0.84 150.0 0.1015 1.23.301.5 0.2360.9 0.5711365±11 1651 ±2317.3 7.1 0.12 591 537 0.94 121.0 0.10280.5 3.37 1.10.238 1.00.874 1376 ±121675±10 17.8 8.1 0.15 336 249 0.76 83.7 0.10840.6 4.33 1.10.290 0.90.817 1639 ±131772±12 7.5 9.1 0.18 544 361 0.69 117.0 0.1040 0.63.581.0 0.2500.9 0.8421439±11 1696 ±1015.2 10.1 – 188 592 3.24 52.5 0.1093 0.74.891.2 0.3241.0 0.8201810±15 1788 ±12–1.2 11.1 0.06 358 365 1.05 97.4 0.10750.5 4.69 1.10.316 0.90.860 1771 ±141757±10 –0.8 12.1 0.43 286 195 0.71 78.1 0.10581.9 4.61 2.10.316 1.00.449 1770 ±151729±35 –2.4 13.1 0.61 517 338 0.68 99.7 0.10110.8 3.11 1.20.223 0.90.728 1297 ±101644±15 21.1 14.1 0.04 409 389 0.98 109.0 0.1085 0.54.641.0 0.3100.9 0.8671742±13 1774 ±9 1.8 15.1 0.27 1021 550 0.56 196.0 0.09810.5 3.01 1.00.223 0.90.858 1296 ±101589±918.4 16.1 0.02 173 96 0.57 48.2 0.10860.7 4.84 1.20.324 1.00.808 1807 ±161775±13 –1.8 17.1 0.05 271 197 0.75 70.0 0.1090 0.64.531.1 0.3010.9 0.8451697±14 1782 ±114.8 18.1 0.03 501 490 1.01 134.0 0.10930.5 4.69 1.00.311 0.90.888 1746 ±131788±82.3 19.1 0.05 311 675 2.24 76.0 0.10770.6 4.22 1.10.284 0.90.840 1612 ±131761±11 8.5 20.1 0.13 519 258 0.51 111.0 0.1058 0.63.641.4 0.2501.3 0.9171437±17 1729 ±1016.8 21.1 0.19 475 453 0.99 104.0 0.10360.6 3.63 1.80.254 1.70.939 1460 ±221690±11 13.6

Note: SHRIMP—sensitive high-resolution ion microprobe. Pbc indicates common lead; Pb* indicates radiogenic lead. Dashes (–) denote that the concentrations of common lead are too low to be precisely measured.

They commonly show weak parallel striations and occur as polysynthetically Zircon selected for U-Pb analysis is subhedral, lamellar to trigonal, and twinned crystals. U and Th concentrations of 15 baddeleyite grains vary from ranges from 30 to 100 mm in length with length:width ratios ranging from 1.2 101 to 241 ppm and from 4 to 12 ppm, respectively. Analyses on 15 baddeleyite to 2.5 (Fig. 5B). Some zircon crystals are striated and also show polysynthetic grains are concordant and yield a weighed mean 207Pb/206Pb age of 1779 ± 8 Ma twinning. U and Th concentrations of 21 zircon grains vary from 173 to 1021 (Fig. 6A). The weighed mean 207Pb/206Pb age of 7 analyses of the Phalaborwa ppm and from 96 to 675 ppm, respectively. The alignment of the discordant baddeleyite standard is 2038 ± 18 Ma. data indicates that these zircon grains belong to the same age group as the

A 0.42 1807 Ma 1771 Ma 1747 Ma 1806 Ma 1791 Ma A

11.1 0.38 2.1 7.1 9.1 3.1 100 ˩m 2000

B 1675 Ma 1758 Ma 1788 Ma 1589 Ma 1774 Ma U 0.34 1900 23 8 / 2.1 7.1 10.1 15.1 14.1 50 m Pb 1800 ˩ 1700

206 0.30 Figure 5. Cathodoluminescence images and ages from the monzonite porphyry (LD03). (A) Repre- 1600 sentative baddeleyite. (B) Representative zircon. Mean 207Pb/P206 b age = 1779 ± 8 Ma, N = 15, MSWD = 1.04 0.26 concordant spots. All data points define an upper intercept age of 1777 ± 17 Ma. The 12 discordant spots plotted below the concordia line with a large discor- dance tend to have relatively high U contents and much younger ages ranging 0.22 from 1772 to 1589 Ma, indicating that the discordant analyses were affected 3.4 3.84.2 4.6 5.05.4 5.86.2 207 235 by Pb loss. Omitting the disconcordant data, the remaining 9 analyses yield Pb/ U a weighed mean 207Pb/206Pb age of 1777 ± 8 Ma (Fig. 6B). The weighed mean 206Pb/238U age of 6 analyses of the TEMORA zircon standard is 416.8 ± 3.5 Ma.

B 1900 0.34 AMPHIBOLE AND PLAGIOCLASE THERMOBAROMETRY Intercept at 1777 f 17 Ma, N = 21, MSWD = 3.2

Mineral Chemistry 1800 1700 Amphibole and adjacent plagioclase pairs were measured rim on rim with 0.30

U 8.1

a JEOL JXA-8800R at the Laboratory of Electronic Probe, Institute of Mineral 8 1600 3.1 23

Resources, Chinese Academy of Geological Sciences. All analyses were con- / 19.1 1.1 ducted under 20 kV accelerating voltage, 20 nA beam current with 5 mm beam 5.1

Pb 1500 size. As the porphyry samples were intensively altered by hydrothermal fluids, 0.26 21.1 20 6 9.1 20.1 amphibole and plagioclase pairs with smooth surfaces were chosen for tem- 1400 perature and pressure estimation. The measured content of main oxides and 6.1 7.1 calculated cation numbers are shown in Tables 2 and 3. Notably, Zr content in 1300 15.1 the altered hornblende ranges from 160 to 1810 ppm (Table 2), with an average 0.22 13.1

value of ~809 ppm. According to the classification of Leake et al. (1997), the 207 206 Mean Pb/ Pb age = 1777 f 8 Ma, N = 9, MSWD = 1.4 hornblendes are classified as ferro-edenite (Fig. 7). without 1.1, 3.1, 5.1, 6.1, 7.1, 8.1, 9.1, 13.1, 15.1, 19.1, 20.1, 21.1 0.18 Pressure-Temperature Estimates 2.4 2.8 3.2 3.6 4.04.4 4.8 5.2 5.6 207Pb/ 235U Previous Al-in-hornblende barometers only considered the influence of Figure 6. Concordia diagrams showing sensitive high-resolution ion microprobe U-Pb data from Al content in hornblende on pressure (P) estimation (Hammerstrom and Zen, the monzonite porphyry (LD03). MSWD—mean square of weighted deviates. (A) For baddeleyite. 1986; Hollister et al., 1987; Johnson and Rutherford, 1989; Schmidt, 1992). (B) For zircon.

TABLE 2. REPRESENTATIVE ANALYSES FOR HORNBLENDES IN THE MONZONITE PORPHYRY SAMPLES USING ELECTRON MICROPROBE AND RESULTS OF THERMOBAROMETRY LD01 LD02 LD03 Sample spot: 123 123456 123456 Major element (wt%)

SiO2 43.7 43.7 43.79 44.52 43.52 43.93 43.87 44 44.39 43.94 43.53 42.29 43.94 42.46 43.72

Al2O3 7.36 7.29 7.53 6.79 7.64 7.07 7.33 7.17 7.34 7.16 7.08 7.93 7.21 8.09 7.36

TiO2 1.11 1.02 1.11 1.04 0.94 1.13 1.25 1.15 1.23 1.06 1.11.011.221.070.91 FeO 23.5 22.36 21.99 21.65 23.4 21.15 21.49 21.75 22.04 21.96 22.66 24.52 21.32 24.31 21.94 MgO 8.22 8.82 8.81 9.23 7.78 9.63 99.299.189.038.557.069.356.778.96 MnO 0.2 0.29 0.25 0.210.170.080.240.230.240.150.170.170.220.150.18 CaO 10.7 10.6 10.48 10.65 10.94 10.85 10.62 10.7 10.68 10.7 10.63 10.77 10.77 10.93 10.63

Na2O 2.00 1.89 1.98 1.681.791.831.951.951.922.061.891.831.851.8 1.92

K2O 1.03 1.22 1.21 1.181.051.221.141.261.241.111.221.341.191.371.20 Total 97.82 97.19 97.15 96.95 97.23 96.89 96.89 97.5 98.26 97.17 96.82 96.92 97.05 96.96 96.82 Trace element (ppm) Zr 160 220 160 430 1020 1180 800 1810 1700 960 350 1290 290 940 820 Calculated cation numbers on the basis of 23 oxygen atoms Si 6.725 6.739 6.723 6.839 6.7116.764 6.747 6.756 6.750 6.764 6.760 6.6116.754 6.636 6.742 AlT 1.335 1.325 1.363 1.230 1.389 1.283 1.329 1.298 1.316 1.299 1.296 1.461 1.307 1.491 1.338 AlIV 1.275 1.261 1.277 1.161 1.289 1.236 1.253 1.244 1.250 1.236 1.240 1.389 1.246 1.364 1.258 AlVI 0.060 0.064 0.086 0.069 0.100 0.047 0.076 0.054 0.066 0.063 0.056 0.072 0.061 0.126 0.080 Ti 0.128 0.118 0.128 0.120 0.109 0.131 0.145 0.133 0.141 0.123 0.128 0.1190.141 0.126 0.106 Fe3+ 0.381 0.392 0.486 0.391 0.541 0.317 0.427 0.306 0.373 0.357 0.323 0.523 0.344 0.455 0.450 Fe2+ 2.643 2.492 2.338 2.391 2.477 2.407 2.337 2.487 2.429 2.470 2.620 2.682 2.397 2.722 2.379 Mg 1.885 2.027 2.016 2.113 1.788 2.210 2.063 2.126 2.080 2.072 1.979 1.645 2.142 1.577 2.059 Mn 0.026 0.038 0.033 0.027 0.022 0.010 0.031 0.030 0.031 0.020 0.022 0.023 0.029 0.020 0.024 Ca 1.764 1.751 1.724 1.7531.808 1.790 1.750 1.760 1.740 1.765 1.769 1.804 1.774 1.830 1.756 NaM4 0.111 0.118 0.189 0.136 0.156 0.088 0.172 0.104 0.139 0.131 0.103 0.133 0.1130.144 0.146 NaA 0.486 0.447 0.400 0.365 0.380 0.458 0.410 0.476 0.427 0.484 0.466 0.422 0.438 0.402 0.428 ϒA 0.312 0.313 0.363 0.404 0.414 0.302 0.367 0.277 0.332 0.298 0.292 0.3110.328 0.325 0.336 K 0.202 0.240 0.237 0.231 0.207 0.240 0.224 0.247 0.241 0.218 0.242 0.267 0.233 0.273 0.236 Total 15.688 15.687 15.637 15.596 15.586 15.698 15.633 15.723 15.668 15.702 15.708 15.689 15.672 15.675 15.664 Fe# 0.62 0.59 0.58 0.57 0.63 0.55 0.57 0.57 0.57 0.58 0.60 0.66 0.56 0.67 0.58 Fe* 0.13 0.14 0.17 0.14 0.18 0.12 0.15 0.11 0.13 0.13 0.11 0.16 0.13 0.14 0.16 P 2.46 2.94 3.62 2.79 3.28 3.24 3.43 3.24 3.45 3.34 3.21 4.09 3.25 4.30 3.57 T 471.6 508.0 557.5 544.9 505.5 568.0 557.5 554.2 573.9 569.3 552.0 546.3 549.7 555.2 575.7 Note: Pressures and temperatures were calculated by iteration of the barometer of Anderson and Smith (1995) with thermometer B of Holland and Blundy (1994). Calculation of ferric iron ratio followed the method of Schumacher (1997). Calculation of cation numbers on certain sites of hornblendes referred to Holland and Blundy (1994). Fe# = (Fe3++ Fe2+)/ (Mg2++ Fe3++ Fe2+) in atomic ratio; Fe* = Fe3+/(Fe3++ Fe2+) in atomic ratio. Abbreviations: M4 and A —certain sites in hornblendes; ϒA —vacancy on the A-site; AlT—total aluminum cation numbers; AlIV and AlVI—aluminum cation numbers of the T1-site and M4-site in hornblendes; P—pressure (kbar); T—temperature (°C).

A revised formula (Equation 1) proposed by Anderson and Smith (1995) in- The initial amphibole and plagioclase thermometer proposed by Blundy corporated the effect of temperature (T ) using experimental data at ~675 °C and Holland (1990) was a semiempirical formula based on the edenite-tremo- (Schmidt, 1992) and at ~760 °C (Johnson and Rutherford, 1989) and was taken lite reaction and often resulted in high temperatures (Poli and Schmidt, 1992). as the proper barometer for pressure estimation of the porphyry in this study. Holland and Blundy (1994) recalibrated the thermometer by extending the data set and taking nonideal mixing both in hornblende and plagioclase into T P(±0.6kbar) =−3.01+ 4.76 Al −{[T (°C) − 675] /85} account. The thermometer A proposed by Holland and Blundy (1994) is also ⋅{0.530 AlT + 0.005294 ⋅[T (°C) − 675]}. (1) based on the edenite-tremolite reaction and is applicable for quartz-bearing

TABLE 3. REPRESENTATIVE ELECTRONIC MICROPROBE ANALYSES FOR PLAGIOCLASES FROM THE MONZONITE PORPHYRY SAMPLES LD01 LD02 LD03 Sample spot 123 123456 123456 (wt%)

SiO2 67.43 67.28 67.1 67.61 67.6966.77 66.8967.19 67.1567.96 66.8767.24 67.1966.51 67.43

A12O3 20.4 21.21 21.64 21.03 19.9321.48 22.1520.72 21.1620.55 20.6220.76 20.7820.91 21.42

TiO2 ––0.03 –0.01– 0.02 0.03 0.08 ––0.05 ––0.03 FeO 0.32 0.3 0.34 0.17 0.08 0.25 0.36 0.18 0.25 0.16 0.19 0.21 0.27 0.30.33 MgO 0.01 0.01 0.02 0.03 0.02 –0.09– 0.01 0.01 0.01 0.01 0.01 0.05 – MnO ––––––––– –0.02– 0.01 –0.04 CaO 0.06 0.14 0.27 0.36 0.15 0.52 0.27 0.35 0.42 0.38 0.34 0.24 0.33 0.34 0.47

Na2O 10.31 10.39 9.85 10.45 10.9610.22 9.33 10.689.919.7910.39 10.3810.45 10.2810.42

K2O 0.1 0.1 0.31 0.08 0.06 0.06 1.09 0.09 0.07 0.07 0.08 0.08 0.08 0.11 0.06 Total 98.63 99.43 99.56 99.72 98.9 99.31100.1999.25 99.0398.92 98.5398.96 99.1298.49 100.2 Anorthite 0.3 0.7 1.5 1.8 0.72.7 1.51.8 2.32.1 1.81.2 1.71.8 2.4 Albite 99.1 98.7 96.6 97.7 98.9 96.9 91.5 97.7 97.3 97.5 97.7 98.3 97.8 97.5 97.2 Orthoclase 0.6 0.6 2 0.5 0.30.4 70.6 0.40.4 0.50.5 0.50.7 0.4 Note: All oxides are measured in wt%. Dashes (–) denote that the concentrations of certain oxides are below the detection limit.

igneous rocks. Holland and Blundy (1994) also formulated the thermometer intergrowths, K-feldspar stringers that emanate from the same seed porphyry based on the edenite-richterite reaction to get thermometer B (Equation 2), show optical continuity with each other and have the same extinction direction. which is also suitable for quartz-free assemblages. We calculated the pres- Quartz grains distributed between feldspar stringers have a parallel, plumose, sures and temperatures by iteration of the barometer (Equation 1) proposed and radiating arrangement; some are round and triangular in different sections, by Anderson and Smith (1995) with thermometer B (Equation 2; Holland and and they all have the same extinction direction. The granophyric textures indi- Blundy, 1994), because Anderson (1996) suggested that this calibration could cate rapid and simultaneous crystallization of feldspar and quartz under mag- yield more reliable results. Results are shown in Table 2. matic undercooling conditions (Vogt, 1921; Dunham, 1965; Barker, 1970; Lof- gren, 1971; Smith, 1974; Fenn, 1986; London et al., 1989; Lentz and Fowler, 1992; M4 M2 T1 A 78.44 +Yab−an − 33.6XNa − (66.8 − 2.92P )⋅X Al + 78.5X Al + 9.4XNa T = (2) Candela, 1997; Lowenstern et al., 1997). Lowenstern et al. (1997) proposed that  M4 T1 plag 0.0721− R ln 27⋅XNa X Si X an elongate K-feldspars grow as cellular, quasi-skeletal forms on preexisting feld- ⋅  M4 T1 plag 64 ⋅X Ca X Al X ab  spar phenocrysts during undercooling with quartz nucleating on the K-feldspar stringers and filling in the interstices, to form granophyric intergrowths. 2 for Xab > 0.5, Yab-an = 3.0 kJ. Otherwise, Yab-an = 12.0(2Xab – 1) + 3.0 kJ. Magmas that ascend close to the surface without eruption can be under- Consequently, this calculation yielded subsolidus temperature estimates of cooled by rapid rising of magma and heat conduction with cold country rocks. 470–580 °C, as shown in the P-T plot (Fig. 8), indicating subsolidus reequilibra- In addition, highly evolved magmas often possess large amounts of dissolved

tion of amphibole and plagioclase in this porphyry during later hydrothermal volatiles (e.g., H2O, F, and Cl; Heimann et al., 2008). Undercooling of magma alteration. may be related to saturation and exsolution of these volatiles with depressur- ization during magma emplacement (Lowenstern et al., 1997; Candela, 1997). DISCUSSION Formation of granophyric texture of the studied sample is indicated by stage II in Figure 9. Formation of Granophyric Texture during Emplacement Hydrothermal Alteration Granophyric texture often occurs in epizonal granitic bodies (Barker, 1970), particularly those associated with extrusive rocks (Buddington, 1959; Dunham, The monzonite porphyry of this study was intensively altered by hydrother- 1965). The granophyric intergrown K-feldspar and quartz form the groundmass mal fluids. Plagioclase was originally oligoclase to andesine in composition, of the monzonite porphyry. These groundmass minerals appear to nucleate on but was retrograded to albite in the studied samples, as confirmed by electron preexisting feldspar phenocrysts (Figs. 2F, 2G, 3G, and 3H). In the granophyric microprobe analyses (Table 3). Albitization of oligoclase involves a fluid phase

ı CaBA 1.50; (Na + K) > 0.50; Ti < 0.50 5 1 Granite solidus LD01 LD01 LD02 LD02 4 Pargasite LD03 Tonalite solidus LD03 (Fe3 +Vİ Al I) Edenite Magnesiohastingsite 3 +VI ) (Fe> Al ) 3 2 + (kbar) P + Fe 0.5 2 Mg /(

Mg Ferropargasite 1 (Fe3 +Vİ Al I) Ferro-edenite Hastingsite 3 +VI (Fe>Al ) 0 450 500 550 600 650 700 750800 850 900 T (°C)

0 Figure 8. Pressure-temperature (P-T ) plot comparing subsolidus conditions of the monzonite 7 6.5 6 porphyry samples with the granite and tonalite solidus. Si in formula

Figure 7. Classification of hornblende according to the nomenclature of Leake et al. (1997). ing Waifangshan, Xiong’ershan, and Xiaoshan (Zhao et al., 2001; Han et al., 2006). Han et al. (2006) proposed that albitization of the Xiong’er volcanic rocks occurred during eruption by comparing these volcanic rocks with the late and that introduces Na and Si and releases Ca and Al (Engvik et al., 2008), and unaltered intermediate-mafic intrusions. Hydrothermal alteration on the stud- could occur either during epidotization, which might cause emigration of Ca ied monzonite porphyry and surrounding volcanic rocks of Xiong’er Group is from plagioclase, or by metasomatism of intermediate rocks by siliceous fluids similar and may be synchronous, and the fluids may be related to the volatiles enriched in Na (Li et al., 1979; Allaby and Allaby, 1999). The introduced epidote released by magma when it ascended to the shallow levels (Han et al., 2006). grains distributed in late-stage quartz veins (Figs. 2C and 3A) further indicate Dissolved halogens (F, Cl) in the hydrothermal fluids that altered the studied that the hydrothermal fluids were enriched in silica. porphyry may have been derived from such volatiles. Hornblende was intensively replaced by chlorite and biotite, and ilmen- Albitization and epidotization commonly occur during medium- to ite associated with hornblende was generally replaced by titanite along its high-temperature hydrothermal alteration (Li et al., 1979; Hu et al., 2004). Our margins (Figs. 3B–3F). Fine-grained apatite crystals tend to be distributed on calculations of subsolidus alteration at 470–580 °C on the studied porphyry the margins of hornblende (Figs. 3B–3D) and show a close spatial relation- samples using amphibole and plagioclase thermobarometry fit well with this ship with the secondary titanite (Figs. 3B–3E). Some needle-like apatite grains hypothesis. Similar alteration and resetting of hornblende and plagioclase cut through hornblende, ilmenite, and the granophyric groundmass (Figs. 3E, thermobarometry was described in Anderson et al. (2012). Hence, our conclu- 3F), implying that it is a late-stage crystallized phase and part of a secondary sion is that the porphyry was altered soon after emplacement by medium- to assemblage of chlorite + biotite + apatite + titanites, as shown by stage III of high-temperature hydrothermal fluids enriched in Si, Na, F, and Cl. Figure 9. Alteration of hornblende can release the necessary Ca for apatite and titanite growth during chloritization. As a significant host of halogens such as Crystallization of Baddeleyite F and Cl, formation of apatite by subsolidus reaction indicates that the hydrothermal fluids may also contain mobile F and Cl. Baddeleyite tends to crystallize under silica-undersaturated conditions, and The monzonite porphyry intruded volcanic rocks of the Majiahe Formation thus is rarely seen in quartz-bearing intermediate to acidic igneous rocks. The nearly simultaneously with their eruption. Hydrothermal alteration including occurrence of baddeleyite in the studied monzonite porphyry cannot be ex- albitization has also influenced Xiong’er volcanic rocks in wide areas, includ- plained by a simple model of fractional crystallization. Schärer et al. (1997)

Mts. Xiao IIbIsolated zircon crystallization ZrQ Si Ttn Ttn Na Fluids Ilm HO2 Bad Zrl Cl F

Bi Figure 9. Schematic diagram showing the Chl general magmatic evolution process of the monzonite porphyry. Zrn—zircon; Bad— +EO $S baddeleyite; Ilm—ilmenite; Ttn—titanite; Zrl—zirconolite; Bi—biotite; Hbl—hornblende; Chl—chlorite; Ap—apatite. II a Granophyric texture IIIHydrothermal alteration formed by undercooling

Hydrothermal fluids

Xiong’er volcanic rocks IBaddeleyite Monzonite porphyry Silicaundersaturated crystallization melt

and Kerschhofer et al. (2000) proposed that baddeleyite megacrysts in kim- Ilmenite crystallized closely associated with early-stage hornblende and berlite were inherited from solid mantle, and later recrystallized. Heaman and occurs as ~3% in the studied monzonite porphyry. Contemporaneously, LeCheminant (2001) suggested that similar U-Pb ages of mantle-derived xeno baddeleyite crystallized early from the original magma at stage I in Figure 9, crystic baddeleyite and groundmass perovskite in an alnöite were due to the implying initial relatively low silica activity. Low silica activity during early- close temporal association between a precursor mantle metasomatic event stage crystallization is also supported by the lack of inherited zircon. The leading to baddeleyite crystallization and generation of the alnöite magma. weighted mean 207Pb/206Pb age (1779 ± 8 Ma) of baddeleyite is interpreted as Baddeleyite has also been reported as a late-stage crystallization product in the primary igneous crystallization age of the porphyry. other magmatic settings (Williams, 1978; Lorand and Cottin, 1987; Heaman and LeCheminant, 1993; Dawson et al., 2001). Crystallization of Isolated Zircon Baddeleyite grains in the studied monzonite porphyry occur as inclusions in most other minerals, indicating that they formed at the early stage of crystal- Zircon occurs commonly in igneous rocks of intermediate to acidic com- lization. Baddeleyite formed under such conditions is much larger than that of position and ranges in size from ~20 to 200 mm (Silver and Deutsch, 1963). rapid cooling in mafic dikes (normally <100m m; Wingate and Compston, 2000; The grain size and elongation (length:width) ratio of zircon is commonly be- French et al., 2002; Schmitt et al., 2010). An inheritance origin of baddeley lieved to reflect rate of crystallization (Corfu et al., 2003). Euhedral, large zircon ite is precluded in this study because these monoclinic baddeleyite grains are crystals with length:width ratios of ~2–4 usually form in early zircon-saturated euhedral, showing similar characteristics of luminescence. Moreover, U-Pb melts (Hoskin and Schaltegger, 2003). Small, needle-shaped zircon grains analyses of baddeleyite are clustered together on concordia and defined a with large elongation ratios (to ~12) usually imply crystallization (Hoskin weighted mean 207Pb/206Pb age that is consistent with that of zircon within error. and Schaltegger, 2003) in rapidly cooled, porphyritic, subvolcanic, high-level

granites and gabbros (Corfu et al., 2003). Late-crystallized zircon crystals are Unlike the late-stage magmatic zircon, the secondary zircon rims in the mostly anhedral because they tend to grow interstitially to earlier formed min- studied porphyry (Figs. 4H–4J) are mostly sporadic, small (generally <5 mm), erals (Scoates and Chamberlain, 1995). and caused no obvious Pb loss for the host baddeleyite given the concor- The isolated zircon grains in the monzonite porphyry are commonly small dant U-Pb analysis results. These secondary zircon rims on baddeleyite are (<100 mm) and anhedral with acicular shapes (Figs. 4D–4G). Some needle-shaped considered to be of hydrothermal origin. Zr could be easily transported as zircon grains are distributed along with elongate K-feldspar or on the interface of halogen or hydroxyl complexes (Rasmussen, 2005; Migdisov et al., 2011; quartz and K-feldspar in the granophyric groundmass. The isolated zircon grains Ayers et al., 2012; Bernini et al., 2013), mainly in the form of hydroxyfluoride 0 0 are best explained as a late-stage and rapidly crystallized phase, suggesting that species ZrF(OH)3 and ZrF2 (OH)2 as recommended by Migdisov et al. (2011). the early-stage magma was zircon undersaturated. It is inferred that saturation Therefore, dissolved F and Cl in the hydrothermal fluids could effectively was only reached late in the crystallization history, because of very low Zr con- promote the dissolution and mobility of Zr and other HFSEs from the host tents and/or high Zr solubility (Hanchar and Watson, 2003). minerals in the monzonite porphyry. Many baddeleyite grains are corroded Apart from baddeleyite and zircon, Zr also occurs in hornblende and ilmen- by secondary zircon overgrowths internally (Figs. 4I, 4J) and must serve ite in the studied porphyry. The average residual Zr content in altered horn- as the dominant but not the sole source of mobile Zr for secondary zircon blende is ~809 ppm, which is significantly higher than the Zr concentrations of growth. Notably, zircon has a very different Th/U ratio compared to the whole-rock compositions (ranging from 235 to 399 ppm; Cui et al., 2011). After parent baddeleyite. fractionation of early-stage Zr-rich minerals, including baddeleyite, ilmenite, In general, hornblende contains highly mobile Zr (Rubin et al., 1993; Fraser and hornblende, Zr concentrations in the residual melt may have been far be- et al., 1997) and ilmenite is also a major carrier of Zr (Bingen et al., 2001; Char- low the saturation value. lier et al., 2007), both of which occur in the studied porphyry and were inten- The granophyric texture formed by undercooling during emplacement, sively altered. Only a few baddeleyite grains included in altered hornblende and the hydrothermal alteration could have taken place nearly at the same have complete zircon coronas. Bingen et al. (2001) documented the contribu- time (Han et al., 2006), as discussed herein. The fact that the early-forming tion of ilmenite to zircon corona formation during granulite facies metamor- baddeleyite was not later dissolved as the magma became silica saturated phism. Significant Zr must have been leached from hornblende and ilmenite also indicates a rapid cooling process of the porphyry magma during emplace- altered by halogen (e.g., F, Cl) rich fluids. The complete subsolidus reaction ment. Considering that zircon solubility decreases with decreasing tempera- related to secondary zircon overgrowth is suggested as follows: ture (Wilke et al., 2012) and increasing silica activity (Watson and Harrison, hornblende [NaCa2 (Mg,Fe)5Si7AlO22 (OH)2 (Zr)n ] + ilmenite [FeTiO3 (Zr)n ] + 1984; Bernini et al., 2013), the isolated zircon grains must have formed at late stage of the monzonite porphyry magmatism during its final emplacement, as baddeleyite (ZrO2 ) + fluid (e.g.,SiO2 ,H2O, Cl,F) = 207 206 shown by stage II in Figure 9. The weighed mean Pb/ Pb age (1777 ± 8 Ma) chlorite [(Mg,Fe)5 Al2Si3O10 (OH8 )] + biotite [K(Mg,Fe)3 AlSi3O10 (OH)2 ] +

of isolated zircon grains is interpreted as emplacement time of the porphyry titanite [CaTiSiO4 (O, OH,F)] + apatite [Ca(PO4 )3 (OH,F,Cl)] + zircon (ZrSiO4 ). (3) and within error agrees with the dating of baddeleyite. With the increasing Zr concentration and higher silica activity of the melt, Zircon Overgrowth we conclude that zircon began to precipitate and nucleate on preexisting baddeleyite crystals to form polycrystalline aggregates. Intergrowths and Zircon overgrowths on baddeleyite can form during lower greenschist to overgrowths of zircon occurred by replacement of baddeleyite accompanied granulite facies metamorphism (Davidson and van Breemen, 1988; Patterson by additional silica supplied by fluids and extra Zr from other Zr-rich minerals and Heaman, 1991; Heaman and LeCheminant, 1993; Wingate et al., 1998; (dominantly hornblende and ilmenite). Lumpkin, 1999; Rioux et al., 2010) and hydrothermal fluid alteration (Heaman and Grotzinger, 1992; Heaman and LeCheminant, 1993; Wingate, 2001). It has been reported that the width of zircon rims increases when baddeleyite CONCLUSIONS is subjected to progressively higher grades of metamorphism (Heaman and LeCheminant, 1993). Zircon rims formed in medium greenschist or higher SHRIMP U-Pb dating of baddeleyite and zircon constrain the initial crys- grade metamorphic mafic rocks are usually wider than 10 mm and may cause tallization and final emplacement age of the studied monzonite porphyry as Pb loss for the host baddeleyite (Davidson and van Breemen, 1988; Patter- 1779 ± 8 and 1777 ± 8 Ma, respectively, which agree within error. Euhedral, son and Heaman, 1991; Heaman and LeCheminant, 1993). In contrast, lower platy baddeleyite crystallized early from the primitive, least-fractionated por- greenschist facies metamorphism or low-temperature fluid alteration forms phyry magma with lower silica activity. Primary zircon formed later during zircon overgrowths that are small (<5 mm) and discontinuous on baddeleyite rapid cooling of the late-stage, more fractionated melt enriched in silica during (Heaman and Grotzinger, 1992; Heaman and LeCheminant, 1993). its final intrusion into volcanic rocks of the Majiahe Formation.

Subsequent medium- to high-temperature hydrothermal alteration pro- Cui, M., Zhang, B., and Zhang, L., 2011, U-Pb dating of baddeleyite and zircon from the Shizhai- duced secondary zircon as intergrowths and overgrowths on baddeleyite. The gou diorite in the southern margin of North China craton: Constrains on the timing and tectonic setting of the Paleoproterozoic Xiong’er Group: Gondwana Research, v. 20, p. 184–193, fluids were enriched in silica, Na, and halogens. Additional Zr from decom- doi:10.1016/j.gr.2011.01.010. position of amphibole and ilmenite by the halogen-bearing fluids also made Davidson, A., and van Breemen, O., 1988, Baddeleyite-zircon relationships in coronitic meta significant contributions to secondary zircon overgrowth. gabbro, Grenville Province, Ontario: Implications for geochronology: Contributions to Min- eralogy and Petrology, v. 100, p. 291–299, doi:10 .1007 /BF00379740. Dawson, J., Hill, P., and Kinny, P., 2001, Mineral chemistry of a zircon-bearing, composite, veined and metasomatised upper-mantle peridotite xenolith from kimberlite: Contributions to Min- ACKNOWLEDGMENTS eralogy and Petrology, v. 140, p. 720–733, doi:10 .1007 /s004100000216. We appreciate the editorial patience and comments of Raymond M. Russo. The comments of Dunham, A.C., 1965, The nature and origin of groundmass textures in felsites and grano- two anonymous reviewers are gratefully acknowledged. This study was financially supported by phyres from Rhum, Inverness-shire: Geological Magazine, v. 102, p. 8–23, doi:10 .1017 the Chinese Ministry of Land and Natural Resources (grant 201311116), the National Natural Sci- /S0016756800053838. ence Foundation of China (grant 41173065), Chinese Ministry of Science and Technology (grant Engvik, A.K., Putnis, A., Gerald, J.D.F., and Austrheim, H., 2008, Albitization of granitic rocks: The 2012FY120100), and the Basic Outlay of Scientific Research Work from the Chinese Ministry of mechanism of replacement of oligoclase by albite: Canadian Mineralogist, v. 46, p. 1401– Science and Technology (grant J1403). 1415, doi:10.3749/canmin.46.6.1401. Fenn, P.M., 1986, On the origin of graphic granite: American Mineralogist, v. 7, p. 325–330. 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