Discovery of Paleoproterozoic rapakivi granite on the northern margin of the Yangtze block and its geological significance

The Huashanguan rapakivi pluton in Zhongxiang, Hubei Province, China, is the first discovered Proterozoic rapakivi pluton in the Yangtze block. Based on field and petrographical observations, a typical rapakivi texture was found in the northern portion of the Huashanguan granitic pluton. Almost all the K-feldspar phenocrysts were round to oval in shape and most had plagioclase coat-ings known as rapakivi phenocrysts. Alkali feldspars and quartz had two or more generations. Petrochemically, the Huashanguan rapakivi granites were characterized as having high values of Si, K, Fe, Th, U, La, Ga, Ce, Sm and LREE, low values of Ca, Mg, Sr, Nb, Y and HREE, and a negative Eu anomaly. These geochemical characteristics of the Huashanguan granites were concordant with typical rapakivi granites, and had an affinity to A-type granites. LA-ICP-MS U-Pb zircon dating also was conducted. The dating yielded a 207 Pb/ 206 Pb weighted mean age of 1851±18 Ma (MSWD =1.2), which represents the age of the pluton emplacement. The age of 803±170 Ma at the lower intercept in the concordia diagram corresponds to the age of a later deformation event which affected the pluton, and suggests that the Huashanguan pluton was influenced by Neoproterozoic thermo-tectonic events after its formation. The discovery of Paleoproterozoic Huashanguan rapakivi granites indicates continental rifting or a post-orogenic extensional event that took place in the Paleoproterozoic in the Yangtze block. These events may be related to the breakup of the Paleoproterozoic Columbia supercontinent.

Rapakivi granite is a special rock type (typical A-type granite) in the crust that shows rapakivi texture [1]. Rapakivi granites are indicative of large-scale extensional tectonic settings, and usually are associated with the breakup of a supercontinent [1][2][3][4][5][6][7]. Rapakivi granites are exposed on the interiors and edges of all Proterozoic cratons worldwide, especially in the Northern Hemisphere. They form a near east-west giant rapakivi granite-anorthosite belt, which extends from the southwest of North America to the North China Platform, via Labrador in Canada, South Greenland, the Baltic Shield and the Sciberian Platform [1][2][3][4][5][6][7][8]. The origin of the belt is considered to be related to the breakup of the Columbia supercontinent [9][10][11][12][13][14]. Thus, rapakivi granites *Corresponding author (email: cqma@cug.edu.cn) are important for understanding the evolution of the Proterozoic lithosphere.

Regional geological setting and intrusive geology
Huashanguan granitic intrusions are, with the total area of 37 km 2 , located in Zhongxiang, Hubei Province, on the northern margin of the Yangtze block ( Figure 1). The intrusions consist of four granitic bodies of variable sizes. They are the Huashanguan, Wangjiapeng, Huachong and Xiaojiawan intrusions, and their areas are 22, 12, 2 and 1 km 2 , respectively. And the intrusions are distributed nearly north-south in the core of an anticline and secondary small anticline in Lengshui town. According to the survey of regional geology [39], the pluton intruded into the Paleoproterozoic Yangpo Formation, equivalent to the upper lithologic formation of the Archean Kongling Complex. Thus, the contact between the intrusions and the overlying Nanhua-Sinian strata is an unconformity, and xenoliths from the Yangpo Formation are visible in the pluton. The main rock types of the Huashanguan and Wangjiapeng intrusions are porphyraceous syenogranite and medium grained biotite adamellite with a few feldspar phenocrysts, and there is a gradual transition between the two rock types. While there is only medium grained biotite adamellite in the Huachong intrusion, and that of the Xiaojiawan intrusion is only porphyraceous syenogranite. Xenoliths with schist, gneiss and plagioclase amphibolite remnants also can be seen in the four intrusive bodies. As a result of dynamic metamorphism, the intrusions and the country rocks suffered a certain degree of deformation 1) .
The rapakivi granites reported in this paper are exposed in the porphyraceous syenogranite unit in the northern portion of the Huashanguan pluton. The sample used for zircon dating was collected at 31°17.261′N, 112°24.607′E ( Figure  1(b). The rock had a porphyraceous texture, and the matrix was medium to coarse grained. The phenocryst was a microcline perthite with a plagioclase rim, and the matrix was microcline perthite, quartz, plagioclase, biotite and other components. The main mineral components were: microcline perthite 65%, plagioclase 10% (mostly changed to sericite), quartz 23%, and biotite 2%. As the scale of the Huashanguan rapakivi granites was limited (<1 km 2 ), we only collected two samples (ZX21-1 and 09ZX03) for analysis. For comparison, we also collected a porphyritic biotite adamellite sample (09ZX01-1) near the Huashanguan rapakivi granite pluton. The porphyritic biotite adamellite was medium grained, and had a few phenocrysts. The main minerals were: microcline perthite 49%, plagioclase 27%, quartz 22%, and biotite 2%.

Rapakivi texture and its petrography
The rapakivi texture was first introduced into the international geological literature in 1980 by Jakob J. Sederholm, a famous Finnish geologist [40]. The name "rapakivi granite" is derived from the Finnish word "rapakivi", which means "crumbly rock" and refers to the distinctive weathering behaviour. The Huashanguan rocks with rapakivi textures are grey white to flesh red with porphyraceous textures. These granites are characterized by the ovoidal shape of their alkali feldspar megacrysts, mantling of many ovoids by oligoclase-andesine shells, and some the ovoids remaining unmantled. The ovoids show two generations of alkali feldspar and quartz, which is consistent with the definition of rapakivi granites [1-3,6,7]. These characteristics show that Huashanguan rocks with rapakivi textures belong to typical rapakivi granites. The diameter of the ovoids generally was between 1 cm and 5 cm, with some individuals were over 8 cm, but the most common size range was 2-3 cm ( Figure  2). The percent volume of alkali feldspar phenocrysts accounted for about 45% of the rock. Most of the phenocrysts consist of 3-5 K-feldspar crystals (Kfs), whose optical orientation differed and the single particle size is 5-10 mm with a flabellate shape. They grew poikiliticly to ovoid shape, from the core outward to the rims (Figure 3(a)). Few of the phenocrysts consisted of single K-feldspar crystals. Plagioclase (pl), quartz (Qtz), biotite (Bi), magnetite (Mt) and other small crystals often were wrapped in the ovoidal K-feldspar phenocrysts. These minerals were embedded unevenly in the host crystal (Figure 3(b)), and showed a tendency toward sphericity. Between the joint surface of the fan-shaped K-feldspar crystals, there were vein fillings of biotite, magnetite, quartz and others (Figure 3(a)). Perthitic structure and cross hatched twins were developed in K-feldspar phenocrysts, which belonged to microcline perthite. Most K-feldspar and quartz coexisted, and some showed micrographic textures (Figure 3(c)). Matrix compositions mainly were alkali feldspar, plagioclase, quartz and biotite. Alkali feldspar and quartz could be divided clearly into two or more generations. The particles of early crystallization were larger, and the later generation crystals were smaller. Figure 3(d) indicates that the quartz sizes were significantly different, and the larger crystals were mostly rounded. Accessory minerals mainly were fluorite, apatite, magnetite, and zircon. These features were similar to that of typical Proterozoic rapakivi granites from Finland, Miyun (Beijing) and other places [1- 10,41,42].
During dynamic metamorphism, most of the rocks suffered from deformation and alteration to different degrees. A few of biotite changed to chlorite, and plagioclase mostly changed to sericite. In the deformation zone, wavy extinctions of quartz, kink deformation of biotite, and brittle fractures and intragranular changes of feldspar were obvious (Figure 3(d)-(f)).

Analytical methods
Samples for major and trace element analyses were crushed to less than 200 microns. The analysis of major elements was conducted with X-ray fluorescence spectrometry (XRF) with a Regaku3080E1-type spectrometer at the Geological Experiment Center of Hubei Province. The accuracy of the sample analysis was 1% [43]. Trace elements were analyzed with an inductively coupled plasma mass spectrometer (ICP-MS, Agilent7500a) at the State Key Laboratory of Geological Processes and Mineral Resources in China University of Geosciences (Wuhan). The accuracy of the rare earth element analyses was higher than 5%, and the analytical precision of the other trace elements was 5% to 15%. Details of the analytical methods and apparatus are reported in a previous literature report [44]. The zircons in sample ZX21-1 were separated by standard heavy mineral separation  techniques, including magnetic and heavy liquid separation. Then, representative zircon grains, transparent and no cracks, were selected and imbedded into epoxy resin sample targets under a binocular microscope. The samples were polished after grinding to the center of the zircon particles, and then Cathodoluminescence (CL) microstructures were observed. On that basis, suitable particles and regions for zircon U-Pb age determination were selected. Zircon CL imaging were done using a Hitachi S3000-N scanning electron microscope, equipped with external Chroma cathodoluminescence made in the GATAN company at the Beijing SHRIMP Centre. Micro-area analysis of zircon for U-Pb isotopic and REE composition analyses was completed using an LA-ICP-MS at the State Key Laboratory of Geological Processes and Mineral Resources in China University of Geosciences (Wuhan). The laser ablation system was a GeoLas 2005 equipped with a 193-nm laser. The spot size of the laser was 32 μm, and the pulse was 10 Hz, with energy of 110 mJ. The ICP-MS was an Agilent7500a, which was produced by the Agilent Company, United States. We used helium as the carrier gas of the erosion material in the experiment, zircon 91500 as external standard for isotope component corrections, and we calibrated elemental content with an NIST610 as the external standard and 29 Si as the internal standard. The original data were analyzed with ICPMSDATACAL (ver 5.8) software processing [45]. Common lead was corrected with the method of Andersen [46]. The weighted mean age of the zircons and the calculation of concordia diagrams were undertaken using the ISOPLOT_3.23 process [47].

Major and trace elements
The major and trace elemental analyses of the Huashanguan rapakivi granites (sample ZX21-1 and 09ZX03) and porphyritic biotite adamellite (sample 09ZX01-1) are listed in Table 1.
to an LREE-enriched pattern. Eu showed a negative anomaly (δ Eu = 0.35-0.63). However, the δEu of the Huashanguan porphyritic biotite adamellites was only 0.07, which was strongly depleted. Huanshanguan rapakivi granites had a feature of high Th (40.0-61.8 μg g -1 ), Rb (249-294 μg g -1 ), Ba (915-1228 μg g -1 ) and other trace elements. Conversely, the Ba content of Huashanguan porphyritic biotite adamellites was very low, only 86.2 μgg -1 . In addition, from the trace element spider diagram ( Figure 5(a)), we also can see that Huashanguan rapakivi granites were similar to Finnish and Miyun typical rapakivi granites. Ba, Th, U, La, Ce, Nd and Sm were relatively enriched, and Nb, Sr, Zr and Y were relatively depleted. However, the Ba content of the Huashanguan porphyritic biotite adamellites was extremely low, indicating the chemical composition of Huashanguan porphyritic biotite adamellites differed from that of Huashanguan rapakivi granites. From the diagram of chondrite-normalized REE patterns ( Figure 5(b)), we can see that the two samples of Huashanguan rapakivi granites displayed similar chondrite-normalized REE patterns with significantly-enriched light REE, fractionated heavy REE, and weakly negative Eu anomalies. These characteristics are very similar to those of Finnish and Miyun typical rapakivi granites.

Zircon U-Pb age and characteristics of rare earth element content
The zircons (sample ZX21-1) of the Huashanguan rapakivi granites mainly were light yellow. The shape of most zircons was semi-cylindrical columnar and fragmentation, and a minority were equant grains. Their lengths ranged from 80 and 200 μm, with aspect ratios of 1.5 : 1 to 2 : 1. The zircon CL images ( Figure 6) show that most of the zircons had magmatic oscillatory zoning, but since Th, U contents of part of the zircons were high, the CL images were relatively dark. Zircon Th/U ratios varied from 0.78 to 1.64, which also is consistent with characteristics of magmatic zircons [51]. Twenty-one individual U-Pb spots were done on 21 zircon grains using LA-ICP-MS, and the results are shown in Tables 2 and 3. Among the 21 points, the signal of point 10 showed fluctuations, and the reverse was not harmonic. Thus, these data were not used. The unharmonious ages of the remaining 20 points gave a concordia line with upper intercept at 1901±45 Ma and lower intercept at 803±170 Ma (MSWD = 2.9) (Figure 7). The unharmonious degree and  Th and U contents of points 9, 12, 18 and 19 were higher than others, and 207 Pb/ 206 Pb concordia ages were significantly lower, which indicates a possible variable proportion of grains that had lost some 207 Pb and 206 Pb. Point 8 was relatively harmonic, while the age was significantly lower. From the CL image, we can see the analysis point was at the edge of the zircon, which may be due to post-recrystallization. After removing points 8, 9, 10, 12, 18 and 19, the 15 remaining higher harmonic degree analyses yielded a 207 Pb/ 206 Pb weighted mean age of 1851±18 Ma (95% confidence limits, MSWD = 1.2; Figure 7), which is consistent with the upper intercept age, representing the emplacement age of Huashanguan rapakivi granite intrusions.
CL images ( Figure 6) show that the zircon oscillatory zoning of the Huashanguan rapakivi granites is vague. It is unknown if this due to the fluids. From the chondrite normalized REE distribution patterns of zircon (Figure 8), we can see that the LREE contents of zircons from Huashanguan rapakivi granites were low, and the Ce positive anomalies were obvious, which indicates an apparently magmatic origin. However, zircons of hydrothermal origin generally are characterized by high LREE content and weak Ce positive anomalies [52,53]. Thus, the zircon REE characteristics of the Huashanguan rapakivi granites were different from that of a fluid origin. The phenomenon of vague zircon oscillatory zoning may be related to metamorphic recrystallization. However, the zircon U-Pb concordia curve (Figure 7) shows that the analyses all plot on or near the concordia line, which indicates that the U-Pb systems of most zircons remained closed. These results indicate that we can obtain reliable 207 Pb/ 206 Pb ages for these samples. From the zircon U-Pb ages we can see that the lead of some zircons was lost. This may be mainly related to metamorphic recrystallization caused by later thermal events (especially the strong Neoproterozoic magmatism), and this process also can cause vagueness of the oscillatory zoning. The lower intercept age (ca. 800 Ma) may represent the time when the rock suffered a later thermal event, which also is consistent with the time of Neoproterozoic tectonothermal events in the Yangtze block, or even over the entire South China area [54][55][56][57][58][59][60][61][62][63].

Rapakivi textures
The origin of rapakivi textures has been a long, contentious geological issue lasting more than 100 years. Haapala et al.
[2] and Rämö et al. [3] have ever discussed this problem. In short, the origin can be divided into two categories: magmatic model and post-magmatic exsolution-component adjustment mode. Current studies have shown that a magma mixing mode and sub-isothermal decompression mode of crystal saturated granitic magma (magma mode) are possible. The magma mixing mode stresses the disequilibrium of the texture resulted by the mixing process of felsic and mafic magmas [64]. The sub-isothermal decompression mode stresses partial resortion of alkali feldspar and quartz, and continued crystallization of plagioclase around the alkali feldspar resulted by magma decompression and slow cooling [65]. This origin model has been confirmed by Eklund et al. [66]. According to petrographic observations, the origin of Huashanguan rapakivi textures also should be that of magma crystallization. For example, from the relationship between minerals, it is evident that K-feldspar phenocrysts enwrap almost all the minerals in the rocks. The small   wrapped minerals show a spherical distribution, which is parallel to the surface of the megacrysts. This indicates that during the growth process of K-Feldspar phenocrysts, the slowly growing minerals congregate at the top of the K-feldspar phenocrysts while they are rotating. Only the convection of magma can meet this condition [67]. This means that rapakivi feldspars of Huashanguan are products of magmatic crystallization. Haapala et al.
[1] defined rapakivi granites as A-type granites characterized by the presence, at least in the larger batholiths, of granite varieties showing the rapakivi textures. S-type or I-type granites with this texture are not within the range of rapakivi granites [2,3]. In fact, rapakivi granites from Late Archaean to Tetiary, including intermediatefelsic to felsic rocks. However, according currently available information, the overwhelming majority of occurrences represent granitic rocks of Proterozoic AMCG assemblages (anorthosite, monzonite, charnockite and rapakivi granites). AMCG assemblages are formed in extensional tectonic settings. It is emphasized that they are related to mantle upwelling, melting and decompression of the upper mantle, and magmatic underplating after cratonization of continental crust [3,7].
Petrographic and geochemical studies indicate that Huashanguan rapakivi granites not only have the typical rapakivi textures, but also can be correlated with A-type granites in their chemical composition, which is consistent with the definition of rapakivi granites [1]. Figure 9 shows  [49,68]. Miyun typical rapakivi granite data are from [7]. The diagrams are from [48].
that Huashanguan rapakivi granites belong to A-type granites of sub-alkaline series. They fall within the Finnish typical rapakivi granite composition field, and are similar to the Miyun rapakivi granites. The major elements of Huashanguan rapakivi granites are rich in silicon, alkalis (especially rich in K) and iron, with high FeO*/MgO ratios, and low calcium and magnesium content. The rocks are rich in Th, U, La, Ga, Ce and Sm, and poor in Sr, Nb and Y, and characterized by LREE enrichment, HREE depletion and a negative Eu anomaly, which also are consistent with the affinity of rapakivi granites. Although we have not found the equivalent of AMCG assemblages in this district yet, there is evidence of contemporary mafic magmatic activity near the Huashanguan rapakivi granites. For example, Peng et al. [37] found contemporary (~1.85 Ga) mafic dikes in the Kongling high-grade metamorphic terrain, not far from the Huashanguan rapakivi pluton in the Yangtze block. This finding indicates the existence of bimodal magmatic associations.
However, Rämö et al. [3] noted that "each rapakivi pluton has its own peculiarities." It is important to understand how these granites differ from the typical rapakivi granites of Finland, Miyun and other places, and why it appears that the Huashanguan rapakivi granitic intrusions are smaller and with few appearance of amphibole. Instead, these granites contain mainly biotite, reflecting a higher water fugacity in the magma chamber than that of other rapakivi granitic plutons. This may be the reason why rapakivi granites in this area have a limited distribution. In addition, Rb/Sr and Rb/Ba ratios of Huashanguan rapakivi granites were relatively higher, which means that the intrusions had experienced a high degree of magma crystallization differentiation.

Tectonic significance of Paleoproterozoic rapakivi granites in the Yangtze block
A series of global super-events took place during the Paleoproterozoic. These events may match the global collisional orogenic and amalgamation events in relation to the formation of the Columbia supercontinent [13,69,70], the rapid growth of continental crust [71,72], and activities of super-mantle plumes [73]. Studies on Paleoproterozoic tectonic magmatic events are relatively few in the Yangtze block, but in recent years, more Paleoproterozoic records of geological events have been described and discussions about the evolution of the geological events, global tectonic setting and their significance have been taken place.
The existence of the Paleoproterozoic Columbia supercontinent has been widely recognized. There are many records of Paleoproterozoic large-scale tectonothermal events associated with amalgamation and breakup of the Columbia supercontinent in the North China block [10,15,24]. The latest research shows that there are many chronological records of the widespread 2.1-1.8 Ga tectonothermal events in the Yangtze block.
(1) 2.1-2.0 Ga magmatic events. Magmatic bodies of this period have not been found in the Yangtze block so far. However, detrital zircon geochronology shows that the Yangtze block has produced a wide range of 2.1-2.0 Ga zircons in the Neoproterozoic sedimentary rocks. They show oscillatory zoning and have high Th/U ratios (~1.0). It is clear that they are of magmatic origins [74,75]. In addition, researchers have obtained ca. 2000 Ma zircon xenocryst ages (using LA-ICPMS, the majority of zircons have Th/U>1) in the lamproite around the Yangtze block [30]. Furthermore, 2091-2025 Ma old zircon ages were measured in Mesozoic zircons of the Tongling district, Anhui Province (SHRIMP method, Th/U is 0.15-0.46, with oscillatory zoning) [76]. These all indicate 2.1-2.0 Ga magmatic events are widely represented in the Yangtze block.
(3) ~1.85 Ga extensional rifting events. Rapakivi granites were formed in an anorogenic or post-orogenic tectonic environment, and are considered to be one of the extensional environmental indicators [1]. The Huashanguan granites reported in this paper belong to the typical rapakivi granites. Dating results show that the pluton was formed at about 1850 Ma in the late Paleoproterozoic. Conversely, the orogenic events related to the amalgamation of the Columbia supercontinent took place at 2.0-1.9 Ga, just before the formation time of Huashanguan rapakivi granite intrusions. In addition, the formation time of mafic dikes, marking regional extension found in the Kongling high-grade metamorphic terrain, also were about 1.85 Ga [37]. Besides, the formation age of the Quanyishang A-type granites recently reported is about 1.85 Ga. Zircon Hf isotopic studies show that the source of Quanyishang A-type granites may come from the Archean crust deep in the Yangtze continent, and that this source may be related to the extension and collapse of the deep crust with Archean ages, in response to the transition stage of the assembly and breakup of the Columbia supercontinent [36]. While the source of the Huashanguan rapakivi granites reported in this paper is unknown, it is possible that it may be from the deep Archean continental crust. Further isotope studies may confirm this assertion. However, the Huashanguan rapakivi granites, Quanyishang A-type granites and mafic dikes in the Kongling high-grade metamorphic terrain were all formed in the extensional tectonic setting, which indicates that they occurred in the tectonic transformation from collision to extension at about 1.85 Ga in the Yangtze block, and may be associated with breakup of the Columbia supercontinent.
In summary, there are many chronological records of the widespread 2.1-1.8 Ga tectonothemal events in the Yangtze block. These may represent the evolutionary history of supercontinent amalgamation to breakup. In other words, they may belong to part of the amalgamation and breakup process of the Paleoproterozoic Columbia supercontinent. The discovery of 1.85 Ga Huashanguan rapakivi granites provides important evidence for the formation and cratonization of the Yangtze block in the Paleoproterozoic, and for its later tectonic transformation from collision to extension. Recently, there have been reports on orogenic events of 1.89-1.83 Ga [84], and breakup events of 1.80-1.76 Ga [85][86][87] in the Cathaysian block. These events took place a little later than those in the Yangtze block, which indicates that the two blocks may have been located in different positions in the supercontinent system. Therefore, they had different evolution histories.

Conclusions
(1) Huashanguan rapakivi granites have typical rapakivi textures. They are characterized by the ovoidal shapes of their alkali feldspar megacryst, being mantled with oligoclase-andesine shells, and both the alkali feldspar and quartz generally show two or more generations. Minerals wrapped in K-feldspar phenocrysts mostly show a spherical distribution. Huashanguan rapakivi granites belong to A-type granites of sub-alkaline, peraluminous, high Fe/Mg ratios and with typical rapakivi texture.
(2) Zircon U-Pb dating shows that the emplacement age of the Huashanguan rapakivi granite pluton was 1851±18 Ma. The lower intercept U-Pb age of 803±170 Ma may represent the time that the intrusion was affected by the later Neoproterozoic tectonic events.
(3) The research shows that there may have been widespread late Paleoproterozoic (ca. 2000 Ma) magmatic and metamorphic events associated with the amalgamation and breakup of supercontinent masses in the Yangtze block. The later development of 1.85 Ga Huashanguan rapakivi granites indicates that the Yangtze block was in a continental breakup or post-orogenic extensional tectonic setting at 1. 85 Ga, which may be related to the breakup of the Paleoproterozoic Columbia supercontinent.