# Multiple Facies Metamorphic Rocks: A Rare Example from the East Qinling Orogen, China
## 5 Discussion
### Crystallisation Temperatures and Possible Metamorphic Path  
Away from tectonic plate boundaries, the temperature increases by about 25 \({}^{\circ}\)C per km depth [31]. The temperature range for the amphibolite-facies is commonly assumed to be 500-750 \({}^{\circ}\)C, corresponding to a crustal depth of about 25 km.  
The Zr-in-rutile thermometer is commonly regarded as a reliable tool for estimating the temperatures at which zircons crystallise, based on the temperature-dependent replacement of Ti by Zr in the rutile lattice [26; 27; 28]. In our studied samples, zircon grains occur within or adjacent to the rutile crystals and quartz forms aggregates in the ore (Figure 4B,C), making possible the application of the Zr-in-rutile geothermometry.  
Three types of Zr-in-rutile thermometers have been proposed for the estimation of the rutile crystallisation temperatures with or without pressure calibrations (Table 4):  
T (\({}^{\circ}\)C) = 127.8 \(\times\) ln (Zr in ppm) \(-\) 10 [26];  
T (\({}^{\circ}\)C) = (4470 \(\pm\) 120)/((7.36 \(\pm\) 0.10) \(-\) log (Zr rutile)) \(-\) 273 [27];  
T (\({}^{\circ}\)C) = (85.7 + 0.473 P)/(0.1453 \(-\) R ln (Zr in ppm)) \(-\) 273 [28].  
The estimated crystallisation temperature for rutile from the Baishugang-Wujianfang district is calibrated as the average values of 593 \(\pm\) 29 \({}^{\circ}\)C (n = 9) and 634 \(\pm\) 50 \({}^{\circ}\)C (n = 9) without pressure, and at a pressure of 7.0 kbar, as 630 \(\pm\) 30 \({}^{\circ}\)C (n = 9) [28; 34; 35] (Table 4).  
Figure 7: Cr versus Nb discrimination diagram for rutils from the Baishugang–Wujianfang districts. Reprinted with permission from ref. [33]. 2018 Springer.  
These temperature estimates are consistent with the metamorphic grade of the region studied (Figure 8). The lower estimated temperatures (630 \({}^{\circ}\)C) are consistent with an intermediate P/T ratio [36]. The higher temperatures are consistent with those estimated by conventional thermometers for the lower unit of the Kuamping Group (i.e., at a pressure of 9.3-11.2 kbar and a temperature of 610-570 \({}^{\circ}\)C) and the group's upper unit being at a pressure of 6.6-8.9 kbar and a temperature of 650-630 \({}^{\circ}\)C [37; 38; 39; 35]. Figure 8 shows the rutile deposits at the Baishugang-Wuijianfang districts are distinct from the metamorphism such as eclogite- and granulite-facies elsewhere in the orogen.  
The rules from the Baishugang-Wuijianfang district coexist with magnetite and have high-Nb contents of 15,000-64,500 ppm (Figures 3 and 4). In addition, no exsolution lamellae of ilmenite and sphene, or chlorite replacing biotite and pyroxene were observed in the samples under reflected light microscopy and in backscattered electron (BSE) images. This indicates that retrograde metamorphism from granulite-facies is not an option.  
We thus suggest that most of the rutile at the Baishugang-Wuijianfang districts are related to prograde metamorphism at temperatures of around 600 \({}^{\circ}\)C corresponding to a crustal depth of \(\sim\)25 km. The rutile deposits at the Baishugang-Wuijianfang districts are distinct from the high-pressure and high-temperature metamorphism such as eclogite- and granulite-related types elsewhere in the orogen [37]. The enrichment of a large amount of rutile formed during prograde metamorphism under medium-temperature and medium-pressure conditions is undoubtedly of great significance for the research of rutile deposit.