Journal of Earth Science  2018, Vol. 29 Issue (5): 1005-1009   PDF    
0
The Origin of Spots in Contact Aureoles and Over-Heating of Country Rock Next to a Dyke
Roger Mason, Rong Liu    
School of Earth Sciences, China University of Geosciences, Wuhan 430074, China
ABSTRACT: Petrographic comparison of andalusite in the contact aureole of the Fangshan pluton, Zhoukoudian, Beijing, China, and hornfels in the aureole of the Markfield diorite, Leicestershire, England, shows that spots characteristic of the outer zones of contact metamorphic aureoles did not form during the progressive stage of contact metamorphism, but are pseudomorphs after earlier andalusite with characteristic chiastolite microstructure. The baked margin produced by contact metamorphism of syenite against a dyke in the Houshihushan ring complex, Shanhaiguan, Hebei Province, China, is an unusual feature caused by the dyke's role as a feeder to a family of cone-sheets.
KEY WORDS: contact metamorphism    contact aureole    retrogression    hornfels    spots    Zhoukoudian    Houshihushan    

1 DEFINITIONS

Contact metamorphic rocks occur at or close to contacts of igneous intrusions where mineralogical and/or microstructural (textural) changes to sedimentary or igneous country rocks appear. The changes are seen over distances of only a few millimeters (mm) up to kilometers (km), depending mainly on the size of the igneous intrusion and composition, texture and structure of the country rocks. The region of contact metamorphic rocks surrounding an igneous intrusion is described as a contact aureole and its surrounding rocks are called country rocks. Field relations demonstrate that the metamorphism of the rocks was caused by heat escaping from igneous magma as it solidified and cooled (see Section 4) and therefore contact metamorphism is sometimes called 'thermal metamorphism', but all metamorphic recrystallization is mainly caused by heat (Treloar and O'Brien, 1998). In some cases a contact aureole can be superimposed on country rocks previously metamorphosed by earlier regional metamorphism. Rocks within a few meters of an igneous contact are often baked to hard fine-grained rock known as hornfels e.g., in the contact aureole of the Fangshan pluton, Zhoukoudian, Beijing, China (Mason and Sang, 2007).

2 PORPHYROBLASTS AND SPOTS IN CONTACT AUREOLES 2.1 Andalusite Porphyroblasts in the Contact Aureole of the Fangshan Pluton, Beijing, China

Figure 1 is a field photo of porphyroblasts of andalusite in phyllite of pelitic (mudrock) composition of the Proterozoic Xiamaling Formation in the almandine-staurolite zone (Chen et al., 2018; Wang and Chen, 1996; Liu and Wu, 1987) of the Fangshan contact aureole less than 600 m from the contact (Binghan Chen personal communication). Andalusite crystals appear as darker prismatic crystals in a finer-grained matrix of phyllosilicates (muscovite and biotite) and quartz. Andalusite, garnet and staurolite grew as new crystals in the contact aureole during the progressive stage of contact metamorphism but are absent in the previously regionally metamorphosed country rock phyllites of the Xiamaling Formation that surround the intrusion. Mason et al. (2010) investigated the growth of andalusite and concluded that the crystals became larger than other minerals because they grew on sparse nuclei but formed relatively quickly by reaction of muscovite with chlorite once the temperature increased above the chemical reaction boundary.

Download:
Figure 1. Porphyroblasts of andalusite in pelitic phyllite, Yangshigou, Zhoukoudian, Beijing, China.

Andalusite crystals display a pattern of inclusions that are diagnostic of a variety of andalusite called chiastolite (Fig. 2). Dark inclusions of graphite form a cross in {001} cross- sections, and cylindrical quartz rods usually 1-3 μm diameter run out from the cross approximately at right angles to {110} faces. Chiastolite only forms in rocks that contain graphite derived from kerogen in the parent mudrock. Andalusite and staurolite display different microstructures in rocks where graphite is absent, containing numerous inclusions of all the matrix minerals (sieve microstructure, texture) and often developing rational faces at their irregular boundaries.

Download:
Figure 2. Chiastolite microstructure displayed by Inclusions in andalusite crystals (Mason et al., 2010, based on Deer et al., 1992).

Harker (1939) recognized the cross pattern of graphite inclusions but not the quartz rods. He saw the rods but did not realize their significance (Fig. 3). Miyashiro (1994) acknowledged his debt to Harker in his books about metamorphic geology. By analysing the microstructure of graphite inclusions in the chiastolite cross and the outer crystal boundary shape, notably re-entrant angles at the corners of crystals, Harker correctly attributed the graphite cross to progressive growth of andalusite by analysing the microstructure of graphite inclusions in the chiastolite cross and the outer crystal boundary shape, notably re-entrant angles at the corners of crystals, but argued incorrectly that it was caused by growth pressure (c.f., Ferguson and Harvey, 1972). In the chiastolite crystal is visible at the bottom of his thin section drawing (Fig. 3a) and quartz rods are visible as lines normal to the {110} faces. He thought they represented predominant {110} cleavage in each sector of the crystal. But Mason et al. (2010) showed that they are quartz rods that grew predominantly normal to the outer {110} faces of the crystal, and suppress one cleavage direction by pegging together the lattice of the host andalusite. They sometimes cross cleavage fractures.

Download:
Figure 3. (a) Chiastolite crystals in an intemediate zone of the Skiddaw aureole, Cumbria, England (from Harker, 1939) showing quartz rods described by Mason et al. (2010) (ppl x23). (b) Chiastolite crystals in chiastolite-cordierite-biotite hornfels of the inner aureole. Cordierite as diffuse oval spots with inclusions (ppl x23). The thin sections that Harker drew are archived in the collection of the Department of Earth Sciences, Cambridge University.

Modern metamorphic petrologists reject the growth pressure mechanism (e.g., Ferguson et al., 1981; Ferguson and Harvey, 1972). Instead, Mason et al. (2010) attributed both the chiastolite cross and the quartz rods to growth of andalusite into a plastic matrix instead by solution and reprecipitation by metamorphic fluid containing a proportion of CO2 as well as predominant H2O (Burton, 1986). The quartz rods do not have crystallographic orientations related to the host andalusite and each rod has its own uniform extinction. Different rods have different extinction angles and some share their orientation with relict quartz inclusions in the chiastolite cross. Deer et al. (1992) do not mention exsolution in andalusite. An exsolution origin may be ruled out. Individual rods keep the same orientation throughout their lengths even when they cross cleavage planes. Andalusite in the Fangshan aureole often shows brittle deformation structures, but not plastic deformation. This rules out internal deformation of andalusite after growth (Mason et al., 2010).

Figure 4 shows an approximately (001) cross section through a chiastolite crystal from the Fangshan Contact Aureole for comparison with Harker's drawing in Fig. 3. The suppression of one cleavage is not evident in this example. Figure 5 is a high power photo of the same specimen showing quartz rods.

Download:
Figure 4. Thin section through chiastolite phyllite displaying (001) section through chiastolite (ppl). Yangshigou, Zhoukoudian, Beijing, China, supplied by Binghan Chen.
Download:
Figure 5. High magnification photo of quartz rods in the chiastolite crystal of Fig. 4. An unusually wide quartz rod is left of the scale bar (ppl).
2.2 Spots in Outer Zones of Contact Aureoles

Harker (1939) emphasised the frequent occurrence of spots in the outer zones of contact aureoles. They do not occur in the Fangshan aureole, which was not discovered at the time he wrote. Mason and Sang (2007) include a thin section drawing of a light coloured spot in spotted slate from the outer zone of the Skiddaw aureole, Cumbria, England. Harker thought that spots formed during the progressive stage of contact metamorphism, and wrote:" They are... produced by reactions involving 'sericite', chlorite and iron-ore. In this case they come in at a rather later stage, and may continue to form, together with biotite until some part of the requisite material is exhausted. As before, the first appearance is as round grains. These are conspicuous by contrast, being relatively free from the biotite- flakes abundantly developed in the rest of the rock, and they thus characterize... [a] type of spotted slate."

Mason et al. (2010) found that andalusite in the Fangshan aureole had undergone several stages of growth under different conditions, followed by a final stage of retrograde metamorphism when andalusite was replaced by fine-grained muscovite, chlorite and chloritoid (Harker's 'sericite'). Biotite inclusions were absent or rare, contrary to expectation if spots formed before chiastolite porphyroblasts. Examination of thin sections of spotted slate from Skiddaw revealed some spots with crystal outlines resembling andalusite. It is more likely that these spots originally formed as andalusite and changed to sericite during retrograde metamorphism that probably occurred during cooling of the contact aureole when it was losing heat to ground water circulating through the country rocks.

2.3 Spotted Hornfels from Cliffe Hill Quarry, Markfield, Leicestershire, England

Some layers of hornfels near a diorite intrusion in Cliffe Hill Quarry, Markfield, Leicestershire, England contain spots (Fig. 6) (Mason and Sang, 2007). This hornfels is light coloured with dark spots (Fig. 7), but in dark hornfelses near other intrusions the spots may be light. The diorite is of late Proterozoic age and was intruded into mm scale laminated Ediacaran sediments that have yielded late Ediacaran fossils and were deposited within the Ediacaran Period (Wilby et al., 2018).

Download:
Figure 6. Spotted hornfels from the contact zone of the Markfield diorite, Leicestershire, England, supplied by Susan Cooke. Note the background scale.
Download:
Figure 7. Schematic sketch of the contact of the Markfield diorite in Cliffe Hill Quarry, Markfield, Leicestershire, England. Susan Cooke supplied the specimen (after Mason, 1978).

Some spots in Fig. 5 grew across the sedimentary layers and some show angular outlines suggesting that they were originally porphyroblasts formed during contact metamorphism. Thin section study (Fig. 8) reveals that the spots are polycrystalline and have a distinct layered structure with a darker rim and a lighter core. The spot in Fig. 7 appears to be a pseudomorph replacing a twinned crystal of andalusite, the larger twin preserving a part of a chiastolite cross as a characteristic dark line of opeque inclusions. A precursor origin, as proposed by Harker (1939), cannot be entirely ruled out but is most unlikely.

Download:
Figure 8. Pseudomorph after andalusite in spotted hornfels, Cliffe Hill Quarry, Markfield, Leicestershire, England (ppl).
3 OVER-HEATED DYKE CONTACT 3.1 Dyke Contact in the Houshihushan Sub-Volcanic Ring Complex, Hebei Province, China

The mountain massif west of Shanhaiguan, Hebei Province, is the remains of an eroded volcano that was active approximately 120 Ma ago, in Early Cretaceous times (Wen et al., 2015) with an outer ring dyke of syenite and an inner boss of alkaline granite containing screens of volcanic rocks erupted from the volcano and brought down to the present erosion level by caldera collapse. All these units and the surrounding country rocks are intruded by inclined cone sheets (Wen et al., 2015; Galland et al., 2014), inclined dykes of porphyritic syenite or alkaline granite between 20 cm and 3 m wide. They have the form of a nested set of cones outcropping radially round the intrusion and dipping towards a common center point below the present land surface. Student parties often visit the southern part of the syenite ring dyke at Yansaihu, where it is intruded by a number of porphyritic microgranite dykes, easily distinguished from the syenite by their darker colour and columnar jointing.

At one locality, a narrow zone of contact metamorphism (BZ) next to a 2 m wide dyke has been caused by heat from a dyke that baked the syenite for 1-2 mm from the contact, changing the colour of the alkali feldspar crystals from pink to grey in an irregular baked margin (Fig. 9). But this example familiar to Chinese students is less straightforward than it seems. Several dykes of similar width cut the syenite nearby but do not have baked margins.

Download:
Figure 9. Contact between syenite of the Houshihushan igneous ring complex, Shanhaiguan, Hebei Province, China (above) and a dyke of granitic porphyry with K-feldspar phenocrysts (below) (photo by Long-Kang Sang, diameter of coin 20 mm).

Thermal conduction modelling of heat transfer from intrusions to the surrounding country rock indicates that when an intrusion fills with magma once and then solidifies, the highest temperature attained in the country rocks adjacent to the contact will only be a little more than half the temperature of the intruded magma (Mason and Sang, 2007). This temperature was not high enough to change the colour of the feldspar crystals. Something unusual happened at the locality shown in Fig. 9.

Only one side of the dyke is visible at the outcrop. Closer examination reveals that two successive intrusions of magma formed two chilled margins, 1st CZ has an irregular contact with the syenite and contains no phenocrysts. It is probably fine-grained devitrified glass. The second CZ has a sharp boundary with 1st CZ and contains smaller K-feldspar phenocrysts than the body of the dyke. Some of them are white rather than pink, suggesting that they remained hot after solidification long enough to lose their colour. The dyke filled with hot molten magma at least twice, and other nearby dykes show as many as four chilled margins. A considerable volume of magma flowed continuously through the dyke bringing in extra heat that raised the temperature in the surrounding 1st CZ and syenite country rock higher than elsewhere, in the same way that hot water circulating through a radiator heats a room. The surface of the radiator reaches the temperature of the water supplied by a heating boiler. The dyke at this locality was a feeder dyke to the surrounding cone sheets and heated previously solid syenite to unusually high temperatures.

4 CONCLUSIONS

Some characteristic polycrystalline spots in contact aureoles grew as andalusite porphyroblasts during progressive metamorphism and were replaced by hydration during retrograde metamorphism of andalusite to muscovite, chlorite and chloritoid. The combination of fine-grained crystals known as 'sericite' can possibly be resolved by a petrological microscope under high magnification and petrologists should avoid using the term. Contacts and chilled margins of dykes need careful examination because dykes containing multiple intrusions can raise the temperature of their country rocks far above the usual values.

ACKNOWLEDGMENTS

Roger Mason thanks Prof. Zhendong You for his first invitation to China in 1986 and much advice, information and guidance ever since. The late Prof. Yujing Han introduced us and acted as my guide and interpreter. The late Prof. Long-Kang Sang played a major role in subsequent research and teaching at China University of Geosciences (Wuhan). Dr. Xia Wen, Ms. Yuejie Li and Ms. Binghan Chen assisted in the preparation of the present paper, Binghan Chen re-drew Fig. 7 and Susan Cooke provided the specimen in Fig. 6. The final publication is available at Springer via https://doi.org/10.1007/s12583-018-0882-5.


REFERENCES CITED
Burton, K. W., 1986. Garnet-Quartz Intergrowths in Graphitic Pelites:The Role of the Fluid Phase. Mineralogical Magazine, 50(358): 611-620. DOI:10.1180/minmag.1986.050.358.06
Chen, N.-S., Chen, B. H., Mason, R., et al., 2018. Using Contact Metamorphic Criteria in Contact Aureole to Preliminarily Discriminate Magma Emplacement Mechanisms of Fangshan Pluton, Beijing. Earth Science, 43(1): 99-108.
Deer, W. A., Howie, R. A., Zussman, J., 1992. An Introduction to the Rock-Forming Minerals, 2nd Edition. Harlow, Essex, Longman
Ferguson, C. C., Harvey, P. K., Lloyd, G. E., 1981. On the Mechanical Interaction between a Growing Porphyroblast and Its Surrounding Matrix. Contributions to Mineralogy and Petrology, 75(4): 339-352. DOI:10.1007/bf00374718
Ferguson, C. C., Harvey, P. K., 1972. Porphyroblasts and "Crystallization Force":Some Textural Criteria:Discussion. Geological Society of America Bulletin, 83(12): 3839. DOI:10.1130/0016-7606(1972)83[3839:PACFST]2.0.CO;2
Galland, O., Burchardt, S., Hallot, E., et al., 2014. Dynamics of Dikes versus Cone Sheets in Volcanic Systems. Journal of Geophysical Research:Solid Earth, 119(8): 6178-6192. DOI:10.1002/2014jb011059
Harker, A., 1939. Metamorphism: A Study of the Transformation of Rocks, 2nd Edition. Methuen, London
Liu, G. H., Wu, J. S., 1987. Metamorphic Zones of the Fangshan Area in Beijing. Bulletin of Chinese Academy of Geological Sciences, 16: 113-137.
Mason, R., 1978. Petrology of the Metamorphic Rocks. Allen and Unwin, London
Mason, R., Burton, K. W., Yuan, Y. M., et al., 2010. Chiastolite. Gondwana Research, 18(1): 222-229. DOI:10.1016/j.gr.2010.03.005
Mason, R., Sang, L.-K., 2007. Metamorphic Geology. China University of Geosciences Press, Wuhan. 153
Miyashiro, A., 1994. Metamorphic Petrology. UCL Press, London.
Treloar, P. J., O'Brien, P. J., 1998. What Drives Metamorphism and Metamorphic Reactions?. Geological Society of London, Special Publication, 138: 1-5. DOI:10.1144/GSL.SP.1996.138.01.01
Wang, F. Z., Chen, N.-S., 1996. Regional and Thermodynamic Metamorphism of the Western Hills of Beijing. Field Trip Guide of 30th International Geological Congress, Beijing (in Chinese)
Wen, X., Ma, C. Q., Mason, R., et al., 2015. Subterranean Origin of Accreted Lapilli in Cone-Sheets of the Houshihushan Sub-Volcanic Ring Complex, Shanhaiguan, China. Journal of Earth Science, 26(5): 661-668. DOI:10.1007/s12583-015-0581-4
Wilby, P. R., Dunn, F. S., Kenchington, C. G., 2018. Shallow-Water Organisms Preserved within Deep-Water Avalonian Assemblage Reveal the Coexistence of More Complex Ediacaran Ecosystems. International Conference on Ediacaran and Cambrian Sciences Joint Meeting. August 12-16, 2018, Xi'an. 36-37