Xenolithic, megacrystic basalt, northwest Hebei province, China

--- postcards from the deep Earth

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Figures 1-2. Views of both sides of a circa 1.5-cm-thick, 21x14-cm, 0.75-kg slab, donated to the Royal Ontario Museum. The original sample was sawn along 3 parallel cuts to yield two fine slices and two end pieces. The author retains the largest mass, while two museums and a geological survey each have a fine display piece of their own. By visual inspection, the xenoliths are largely olivine plus a brighter green (?) chrome diopside. The pyroxenes that are present are accompanied by spinels (the latter just 0.5-2.6 wt.% in 16 samples: Princivalle et al., 2014). The sample is from the Damaping village locality, near Zhangjiakou City, Hebei province, China. The principal geological and mineralogical interests here lie in basalt containing upper-mantle peridotite inclusions: ultramafic xenoliths ("nodules") in a vesicular basalt host rock (the vesicles are the white rounded gas bubbles, infilled by white secondary minerals deposited as the lava cooled).

Some additional description:

  • Size: 23x18x22 cm prior to cutting.
  • Mass: very roughly 7 kg prior to cutting, now 3381 (big end), 1480 (Museu end), 1041 (OGS slice) and 753 grams (ROM slice), total circa 6655 grams.
  • Xenolith dimensions: circa 5 to 10 cm and more in maximum dimension. Those examples from Damaping, studied by Rudnick et al. (2004), were 10-35 cm wide, all spinel-facies peridotites (lherzolite to harzburgite, depending on clinopyroxene content).
  • Megacryst maximum dimensions: at least 8x3x2 cm. A Raman spectroscopic test confirms that these coarse, angular, somewhat embayed xenocrysts are clinopyroxene of broadly augitic composition. Both slices contain large sections of these megacrysts. Xenocrysts analysed by Brizi et al. (2003) are reported as subcalcic augite (Wo34) exsolved into pigeonite (En62Fs25-21Wo13-17) and augite (En46-45Fs13Wo40-42), following initial crystallization at mantle depth from alkaline melts.
  • Magnetism: measured magnetic susceptibility (3 measurements per lithology) circa 9.7x10-3 SI units (peridotite) and 16x10-3 SI units (host basalt). These values are not unusual for fresh peridotite and basalt, and equate to a degree of magnetism that responds, in a weak but definite manner to an old-fashioned pen magnet (the basalt markedly more than the xenoliths). Altered peridotites may actually be more magnetic, because the process of serpentinization destroys olivine, generating sheet silicates and also fine-grained secondary magnetite.

"Rock of the Month #217, posted for July 2019" ---

Xenolithic basalt from northwest Hebei province:

This sample displays two lithologies, one entrained within the other. The host rock is a very fine-grained basalt with 0.5-1 mm (rarely 3 mm) vesicles lined with white (?) zeolites, calcite, and/or quartz. It contains abundant rounded to angular (fragmented) xenoliths of coarser upper- mantle ultramafic rock, dunite to peridotite, composed largely of olivine plus lesser pyroxenes and other minerals, grain size 2 mm, individual blocks to 10 cm (or occasionally more) in maximum dimension. The blocks of foreign rock ("xenoliths") are fragments of the upper mantle, carried up to the surface in a younger melt, a basaltic magma, in late Cenozoic time.

Zhangjiakou is some 160 km due NW of the city of Beijing, at around 40°46'N, 114°53'E, at circa 700 m elevation, amidst rugged hills 200-300 m higher. It is a substantial city in its own right, with a regional population over 4 million. It was known as Kalgan before the post-imperial, mid-20th century revolution, and serves as a strategic link from Inner Mongolia, the edge of which lies just is a short distance to the northwest, and Beijing. Located in the Yin Mountains, the region contains coal and iron resources. The province of Hebei is ring-shaped, all but encircling the great municipalities of Beijing and its port city, Tianjin.

A point of geographic trivia: a great-circle route sometimes used by jet liners to fly from Toronto to Beijing passes over or close to several Rock of the Month localities. These include segments of the Grenville province and Abitibi greenstone belt, Baffin Island, the Arctic Russian mining town of Noril'sk, and Kalgan (as it is still called on some maps).

The Mindat database (Damaping) notes the presence of olivine, three pyroxenes (enstatite, augite, pigeonite), spinel and calcite in the Damaping peridot mine, Hannuoba basalt field, Wanquan County. It gives an estimated location some 10 km south of Chongli and 40 km ENE of Zhangjiakou.

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Figures 3-4. Two views of the Damaping location, courtesy of CARMA doctoral student Xiao Li Guanlong. The impressive quarry face displays a basalt crowded with (very roughly) 50 volume percent cm- to dm-scale peridotite xenoliths.

Local geology

There are ancient rocks in the eastern North China Craton, including sheared gneiss remnants in granites near Anshan, Liaoning province, dated at 3800 Ma (Liu et al., 1992). Eastern and western parts of the craton appear to have fused together along a suture zone near Damaping at about 1900 Ma.

Mantle xenoliths are widespread in Cenozoic alkali-rich lavas of eastern China. from Yunnan, Hainan Island and Zhejiang, north through Shandong and Hebei to Inner Mongolia and "Dongbei", the three "Manchurian", northeasternmost provinces of Liaoning, Jilin and Heilongjiang (Cao and Zhu, 1987). There are also granulites and pyroxenites in the suite, that represent the lower crust (Liu et al., 2001).

Nodule lithologies include essentially anhydrous spinel peridotite (Song and Frey, 1989; Zheng et al., 2001; Rudnick et al., 2004), garnet lherzolite and garnet websterite. The basalts are of late Oligocene to Miocene age, erupted at some time between 24.5 to 10 Ma, together with their entrained xenoliths of lower crust and upper mantle materials. The lavas are intercalated alkalic and tholeiitic basalts, with alkali basalts at the base of most profiles (Princivalle et al., 2014). The xenoliths include abundant spinel lherzolites and harzburgites, with rare spinel-garnet lherzolites plus spinel and garnet-bearing pyroxenites and mafic to felsic granulites. The mantle under the Hannuoba basalt field may have undergone partial melting at about 1900 Ma, during collision of the eastern and western blocks of the North China Craton.

Mindat gives the following note on the site: "Accumulation zone of spinel peridotite mantle nodules enclosed in Cenozoic alkali basalts of the Hannuoba basalt field, which also contain abundant clinopyroxene xenocrysts up to 10 cm in size. The mine also works adjacent placers". Published research on the locality (in English) includes aspects of the petrology (Zheng et al., 2001; Rudnick et al., 2004), mineralogy (Brizi et aL, 2003) and isotope geochemistry of the nodule suite. The latter studies include carbon (Liu et al., 1998), lithium, iron, strontium, neodymium and osmium. Ion microprobe (SIMS) data on lithium contents and Li isotope ratios in coexisting silicate grains are interpreted in terms of reaction of peridotite with melt in the lithospheric mantle beneath the northern North China craton. (Tang et al., 2007). Chalcophile elements in the nodules, such as arsenic, zinc and copper, are typically in sulphide-melt inclusions, usually dominated by pentlandite, with lesser chalcopyrite and pyrrhotite, though sulphide blebs in the pyroxene megacrysts are largely pyrrhotite (Xu and Chu, 1998).

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Figures 5-6. The xenolithic basalt in situ, and the original sample, before cutting. Note: individual xenoliths may be rounded, equant, or somewhat flattened in profile. Thus the proportions of xenolith and matrix in a sawn face may well differ from their ratio seen on the outer surface: in some cases, beauty really is skin deep!

Acknowledgements: The sample was kindly donated by Prof. Yang Jingsui on Monday, 23 April 2018. Prof Yang is director of CARMA, the Center for Applied Research on the Mantle, Key Laboratory of Deep-Earth Dynamics, Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, in Beijing. Sawn by Ian Nicklin at the Royal Ontario Museum, where Veronica Di Cecco performed a brief test to identify the megacryst mineral. The smaller end piece is now at a museum in Brazil (the Museu Joias da Natureza in Santos), the slices at the Royal Ontario Museum and the Ontario Geological Survey (Thunder Bay office).

References (n=11, chronological order)

Cao,R-L and Zhu,S-H (1987) Mantle xenoliths and alkali-rich host rocks in eastern China. In `Mantle Xenoliths' (Nixon,PH editor), Wiley-Interscience, John Wiley and Sons Ltd, 844pp., 167-180.

Song,Y and Frey,FA (1989) Geochemistry of peridotite xenoliths in basalt from Hannuoba, eastern China: implications for subcontinental mantle heterogeneity. Geochimica et Cosmochimica Acta 53, 97-113.

Liu,DY, Nutman,AP, Compston,W, Wu,JS and Shen,QH (1992) Remnants of ≥3800 Ma crust in the Chinese part of the Sino-Korean craton. Geology 20 no.4, 339-342.

Liu,G, Wang,X and Wen,Q (1998) Carbon isotopic composition of mantle xenoliths in alkali basalt from Damaping, Hebei. Chinese Science Bulletin 43, Issue 24, 2095–2098.

Xu,J and Chu,X (1998) Sulfide-melt inclusions in mantle xenoliths of Hannuoba, China. Abs. International Mineralogical Association 17th General Meeting, 38, Toronto, 9-14 August.

Liu,Y-S, Gao,S, Jin,S-Y, Hu,S-H, Sun,M, Zhao,Z-B and Feng,J-L (2001) Geochemistry of lower crustal xenoliths from Neogene Hannuoba basalt, North China craton: implications for petrogenesis and lower crustal composition. Geochimica et Cosmochimica Acta 65, 2589-2604.

Zheng,J, O'Reilly,SY, Griffin,WL, Lu,F, Zhang,M and Pearson,NJ (2001) Relict refractory mantle beneath the eastern North China block: significance for lithosphere evolution. Lithos 57, 43-66.

Brizi,E, Nazzareni,S, Princivalle,F and Zanazzi,PF (2003): Clinopyroxenes from mantle-related xenocrysts in alkaline basalts from Hannuoba (China): augite–pigeonite exsolutions and their thermal significance. Contributions to Mineralogy and Petrology 145, 578-584.

Rudnick,RL, Gao,S, Ling,W, Liu,Y and McDonough,WF (2004): Petrology and geochemistry of spinel peridotite xenoliths from Hannuoba and Qixia, North China craton. Lithos 77, 609-637.

Tang,Y-J, Zhang,H-F, Nakamura,E, Moriguti,T, Kobayashi,K and Ying,J-F (2007) Lithium isotopic systematics of peridotite xenoliths from Hannuoba, North China Craton: implications for melt-rock interaction in the considerably thinned lithospheric mantle. Geochimica et Cosmochimica Acta 71, 4327-4341.

Princivalle,F, De Min,A, Lenaz,D, Scarbolo,M and Zanetti,A (2014) Ultramafic xenoliths from Damaping (Hannuoba region, NE-China): petrogenetic implications from crystal chemistry of pyroxenes, olivine and Cr-spinel and trace element content of clinopyroxene. Lithos 188, 3-14.

Graham Wilson, 23-24,26-28 December 2018, last update 21 April 2019, 04 September 2019

See also:
Peridotite nodules in basalt, Victoria, Australia
or visit the "Rock of the Month" Archives!
or browse by category in the
"Rock of the Month Index"
(specimens related to China, and Beijing, appear below). Sites are:
CAGS = China Academy of Geological Sciences, Beijing
CUGB = China University of Geosciences, Beijing (Grounds and Yifu Museum)
NGMC = National Geological Museum of China, Beijing
TGSL = Turnstone / Wilson collection
Various = Other private collections
YMY = Yuanmingyuan, Old Summer Palace, Beijing

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The "Rock of the Month"
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Arsenic ore minerals --- #180 --- Arsenic sulphides, realgar and orpiment, from (?) Hunan, China CUGB
Superb crinoid fossils --- #181 --- Traumatocrinus, exceptional crinoid fossil from Guizhou, China NGMC
Beryl, beryllium cyclosilicate, gemstone --- #186 --- Prismatic beryl from (?) Yunnan, China CUGB
Vertebrate fossil, historically significant --- #201 --- Mesosaurus, fossil reptile & mascot for Gondwanaland (Brazil, via Guangxi, China) CUGB
Ornamental carving stone, China --- #203 --- Qingtian stone, superb lapidary material from Zhejiang, China CUGB
Ophiolitic chromitite --- #205 --- Chromitite, Luobusa ophiolite, southern Tibet (Xizang, China) CAGS
Nephrite jade --- #207 --- Massive jade as decorative piece, from China Various
Peridotite xenoliths in basalt --- #217 --- Mantle nodules and megacrysts, Hebei, China TGSL / CAGS
Tempestite dolostone of Jixian age --- #219 --- Tempestite with algal mats, Tianjin, China CUGB / YMY