SUBDUCTION OF THE MONGOLIAN PLATE

Subduction of the Mongolian oceanic plate during Caledonian time (about 555-396 Ma) may have been a mechanism for repeated heating, fracturing, and hydrothermal mineralization at Bayan Obo (Chao and others, 1995). The tectonic setting of the region that includes the Bayan Obo mine area was discussed by Zhao and Li (1987), Wang and others (1991), and Tang and Yan (1993). These three reports do not fully agree in either description or available isotopic age data. As a consequence, the history and ages of tectonic development and regional metamorphism of Inner Mongolia remain obscure. Zhao and Li (1987) outlined a five-stage plate tectonic model for the North China cratonal margin in Inner Mongolia during the Paleozoic (fig. 34). As a result of oceanic spreading, the Mongolian oceanic plate moved southward during the Paleozoic, causing two episodes of subduction of the Mongolian oceanic plate underneath the North China cratonal margin. The earliest subduction began at the end of the Xin Kai stage of the late Early Cambrian and possibly extended to the early Caledonian. This period of subduction shortened the eugeosynclinal sedimentary sequence of the Xin Kai stage and was accompanied by continental granitic intrusions. The second phase of subduction was near the end of the Middle Silurian and consisted of high-angle rapid subduction of the Mongolian plate, resulting in formation of the Bainaimiao island arc and accompanying island-arc granitic rocks.

Figure 34. Schematic diagram proposed by Zhao and Li (1987), showing five stages of north to south subduction of the Siberian oceanic plate under the northern margin of the North China craton.

Wang and others (1991) described, as resulting from subduction, a 1,800-m-thick ophiolite unit that extends east-west for a distance of about 100 km at Ondor Sum (Wenduermiao), about 275 km east-northeast of Bayan Obo (fig. 4). This ophiolite unit consists, in ascending order, of 200 m of metamorphosed harzburgite, 400 m of cumulate gabbros, 200 m of sheeted diabase dikes, 500 m of basic pillow lavas, and 500 m of abyssal sedimentary rocks. Wang and others (1991) reported that fossils found in the abyssal sedimentary rocks are late Paleozoic to Cambrian in age, the K-Ar age of amphibole in metagabbro is 632-626 Ma, and the metamorphic age of blueschists in the ophiolite series is 435+-61 Ma. The ophiolite sequence is unconformably overlain by Upper Silurian beds containing coral fossils.

Two types of granitic rocks are associated with the gabbroic rocks of the ophiolite sequence found in the Ondor Sum region between Tulinkai and Deyenqimiao. One type consists of small bodies of grayish-white, very fine grained plagioclase granite, containing about 60 volume percent of oligoclase-andesine plagioclase feldspars, about 30 volume percent quartz, 5 percent K-feldspars, 1 percent biotite, and accessory amounts of zircon and magnetite (Zhao and Li, 1987). The other type consists of rare exposures of granophyre associated with pillow lavas in the ophiolite suite. The granophyres are red and fine grained, having intergrowths of K-feldspar and quartz (15-20 volume percent), plagioclase with Carlsbad twins, and about 5-7 volume percent small scattered grains of quartz. The granophyre also contains 5-10 volume percent of dark minerals, mainly epidote and chlorite, and the accessory minerals magnetite, hematite, apatite, zircon, and rutile.

Tang and Yan (1993) compiled geologic and tectonic sketch maps of central Inner Mongolia and a model for the Proterozoic to middle Paleozoic plate tectonic evolution of the Inner Mongolian suture zone. They also described the ophiolite suite of the Ondor Sum (Wenduermiao) region and interpreted it to be a residual fragment of early Paleozoic oceanic crust. According to their description, the ophiolite consists of a lower (>700-m-thick) gabbro and diabase, a middle sequence of sheeted dike swarms and pillow lavas, and an upper sequence of tuffaceous sediments and siliceous-argillaceous rocks. Tang and Yan further reported the metabasalt as having a Rb-Sr (whole-rock isochron?) age of 509+-40 Ma. However, east of Ondor Sum, in the Kedanshan-Wudaoshimen district on the northern side of the Xar Moron River, they found many ophiolite blocks containing chert with Ordovician microfossils.

Although more isotopic age data are needed, the simple model proposed by Zhao and Li (1987), slightly modified and reproduced in figure 34, provides a hypothetical mechanism for repeated heating, fracturing, and hydrothermal mineralization that accounts for the episodes of REE mineralization of the giant Bayan Obo REE-Fe-Nb ore deposit. The Caledonian subduction can be traced for an east-west distance of a little more than 10° longitude, or approximately 850 km. The extent of Caledonian subduction is of interest with regard to other possible giant polymetallic ore deposits that are known or reported to occur along the northern margin of the North China craton. The more extensive Hercynian subduction occurred north of the trace of Caledonian subduction (fig. 4) and was accompanied by widespread granitic intrusion. The importance of the Hercynian subduction with regard to metallogenic ore deposits has not been demonstrated.

The history of intermittent, episodic REE mineralization, based on the general paragenetic sequence and ages of mineralization, would require repeated and alternating periods of fracturing (tensional tectonics) and compression (regional metamorphism). Compression was required to produce heating and generation of hydrothermal solutions to scavenge and concentrate REE's from the deep crustal source area. Fracturing was required to provide channels for the hydrothermal solutions to ascend to near the surface to produce the giant Bayan Obo REE deposit.

GENERATION OF THE CALEDONIAN HEJAO PLUTONIC ROCKS AND THE SOURCES OF METAL-BEARING HYDROTHERMAL SOLUTIONS

An east-trending belt of granitic plutonic rocks about 20-30 km wide and at least 120 km long, about 50 km south of Bayan Obo (fig. 1) has a tentative Paleozoic (Caledonian) age based on the age of similar granitic rocks overlain by Silurian sedimentary rocks north of the Kuanggou fault. According to Zhao and Li (1987), these granitic rocks follow a deep major fault-fracture zone. They intruded metamorphic rocks of the Bayan Obo Group and were, in turn, intruded by Hercynian granitic rocks. The only petrographic description available for the rocks in this belt is that of Zhao and Li (1987), which shows the rocks to consist mainly of granodiorite, monzonitic granite, and quartz biotite diorite. The plutonic rocks are chiefly grayish to pale yellowish, fine to medium grained, and foliated and have poorly developed granular texture. Mineral compositions vary widely; the rocks consist of 45-65 volume percent plagioclase (oligoclase-andesine), 0-30 volume percent K-feldspar (perthite), a few to 30 volume percent quartz, and 2-10 volume percent mafic minerals (biotite or hornblende). Accessory minerals are magnetite, ilmenite, zircon, apatite, garnet, and tourmaline.

We collected several samples of pink, medium-grained quartz monzonite or monzonitic granite (for example, sample 8B53-1) from outcrops near Hejao (fig. 1). They contain fresh black hornblende 0.5-1 cm across and scattered small zircon crystals. The 39Ar incremental heating release pattern of hornblende from sample 8B53-1 shows a plateau age of 455.0+-3.4 Ma (fig. 35; J.E. Conrad and E.H. McKee, unpub. data, 1990). Thirteen small zircon crystals were also separated from this sample. The best estimated U-Pb age of the zircon, 451+-17 Ma (see details in Wang and others, 1994), is in good agreement with the 40Ar/39Ar age of Ordovician (Caledonian) hornblende.

Figure 35. The 39Ar release pattern of incremental heating of hornblende from a quartz monzonite rock, sample 8B53-1, collected near Hejao, showing a plateau 40Ar/39Ar Caledonian age of 455.0+-3.4 Ma (J.E. Conrad and E.H. McKee, unpub. data, 1990).

Zhao and Li (1987, table 1) also published several chemical analyses of the granitic rocks of Hejao. Among them are two analyses with high total alkalis (values in wt. percent):

	Sample number
	513	415
SiO2	77.31	75.72
TiO2	.05	.16
Al2O3	11.47	13.26
Fe2O3	.37	.15
FeO	.41	.45
MnO	.02	.02
MgO	.35	.33
CaO	.19	.51
Na2O	1.82	1.88
K2O	7.27	6.60
P2O5	.00	.00
Loss on ignition	.24	.55

According to Eby (1990), A-type (denoting either anorogenic or anhydrous) granitoids are characterized by their relatively high alkali contents, low CaO contents (at SiO2>70 percent; Na2O+K2O=7-11 percent, and CaO<1.8 percent), and high FeO/MgO values. A-type granitic rocks were emplaced into non-orogenic settings, both within crustal plates and along plate margins, during the waning stages of subduction-zone-related magmatism. The above chemical compositions of Hejao granite samples 513 and 415 meet the criteria of high total alkalis at SiO2 greater than 70 percent, and low CaO content, but they do not meet the criterion of high FeO/MgO values. Zhao and Li (1987) classified three other granitic rocks as S-type granites. On the basis of the average delta 18O value of 10.55 per mil for whole-rock analyses of three Hejao types of granitic rocks, Zhao and Li concluded that they are characteristic of continental crustal origin. The Hejao granitic rocks, however, contain a total REE content ranging from 36 to 58 ppm (Zhao and Li, 1987) and are unrelated to the hydrothermal sources rich in REE's for the REE mineralization of the Bayan Obo ore deposit.

The Hejao granitic plutonic complex should be studied in greater detail with regard to field occurrences, petrology, geochemistry, and isotopic geochronology. We suggest that the Hejao granitic plutonic rocks, an age of 455+-3.4 Ma, and the hydrothermal solutions that were enriched in REE's could have been related to Caledonian subduction of the Mongolian-Siberian oceanic plate beneath the North China craton that occurred at 555-395 Ma, as outlined by Zhao and Li (1987) and Tang and Yan (1993). The crustal sources for the Hejao granitic rocks and the REE hydrothermal solutions may have been quite different because the Hejao granitic rocks are about 50 km south of the Bayan Obo mine region.