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Early Precambrian mafic dykes in western Shandong province, North China Craton: constraints on the chronology, genetic model and tectonic significance

Published online by Cambridge University Press:  02 May 2023

Shen Liu*
Affiliation:
State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China
Caixia Feng
Affiliation:
State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China
Guangying Feng
Affiliation:
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
Yan Fan
Affiliation:
State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China
Zhuang Guo
Affiliation:
State Key Laboratory of Continental Dynamics and Department of Geology, Northwest University, Xi’an 710069, China
*
Author for correspondence: Shen Liu, Email: liushen@nwu.edu.cn
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Abstract

In the Late Neoarchaean, the lithosphere of the North China Craton (NCC) experienced a strong extensional event, which is of great significance for understanding the evolution of the continental crust in the Precambrian. In this study, a suite of mafic dykes from Shandong province in the northeastern NCC were investigated to determine the nature, timing and source of rift-related magma activities using zircon U–Pb data, whole-rock geochemistry and Nd–Hf isotopes. Zircon U–Pb dating of four dolerites by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS) yielded weighted mean 207Pb/206Pb ages in the range 2509 ± 6.1 to 2537 ± 6.2 Ma (2σ, 95 % confidence interval). The mafic dykes are classified as alkaline rocks based on their K2O + Na2O contents (6.78–7.21 wt %) and belong to the shoshonitic series according to their K2O contents (3.23–3.36 wt %). The dolerites show low concentrations of light rare earth elements ((La/Yb)N between 7.17 and 8.55), positive Eu anomalies (Eu/Eu* between 1.12 and 1.27), positive Ba, K, Pb, Sr, Eu, Dy and Lu anomalies, and depleted U, Nb, Pr, Ta, P, Nd and Ti anomalies. The dykes are characterized by low initial (87Sr/86Sr)i (∼0.6969), positive εNd(t) values (0.2–0.8) and εHf(t) values (0.5–8.6) and relatively old mean Nd and Hf model age (2.73 Ga). Collectively, the data suggest that the mafic dykes were derived from the partial melting (10–20 %) of an isotope-depleted garnet–lherzolite mantle source that was hybridized through interaction with subducted lower crustal material. The parental magmas of these dykes underwent a certain number of crustal contaminations during magma ascent. The mafic magmatism represented in the form of the dyke swarms is considered to be a response to widespread lithospheric extension which affected the NCC at c. 2.5 Ga during the Neoarchaean.

Information

Type
Original Article
Copyright
© The Author(s), 2023. Published by Cambridge University Press
Figure 0

Fig. 1. (a) Location of the study area within the northeastern NCC. (b) Geological map of the study area, showing the distribution of mafic dykes studied and the location of samples collected during this study.

Figure 1

Table 1. Zircon LA-ICP-MS U–Pb isotope data for the mafic dykes studied within western Shandong province, northeastern NCC

Figure 2

Table 2. Major element concentrations (wt %) for the mafic dykes studied. LOI = loss on ignition; Mg# = 100Mg/(Mg + Fe) in atomic proportions; RV = recommended values; MV=measured values. Values for GSR-1 and GSR-3 are from Wang et al. (2003)

Figure 3

Table 3. Trace element compositions (ppm) of the mafic dykes studied. RV = recommended values; MV = measured values. Values for GBPG-1 and OU-6 are from Thompson et al. (2000) and Potts and Kane (2005), respectively.

Figure 4

Table 4. Sr–Nd isotopic compositions of the mafic dykes studied. The compositions were calculated using Chondrite Uniform Reservoir values and decay constants of λRb = 1.42 × 10–11 year–1 (Steiger & Jäger, 1977) and λSm = 6.54 × 10–12 year–1 (Lugmair & Harti, 1978)

Figure 5

Table 5. Hf isotopic compositions of representative samples studied in western Shandong province

Figure 6

Fig. 2. The field and microscopic photos of the mafic dykes studied. Pl – plagioclase; Cpx – clinopyroxene.

Figure 7

Fig. 3. Representative CL images of zircon grains from samples from the mafic dykes studied.

Figure 8

Fig. 4. Zircon LA-ICP-MS U–Pb concordia diagrams and the 207Pb/206Pb weighted mean ages from the mafic dykes studied.

Figure 9

Fig. 5. Classification of the mafic dykes studied. (a) Total-alkali silica (Middlemost, 1994; Le Maitre, 2002) using major element concentrations recalculated to 100 % volatile-free compositions. (b) variations in K2O vs Na2O concentrations (Menzies & Kyle, 1972).

Figure 10

Fig. 6. Variations in major element concentrations vs MgO ( wt %) for the mafic dykes studied.

Figure 11

Fig. 7. Variations in trace element concentrations vs MgO ( wt %) for the mafic dykes studied.

Figure 12

Fig. 8. (a) Chondrite-normalized REE and (b) primitive-mantle-normalized multi-element variation diagrams for the mafic dykes studied. Concentrations are normalized to chondrite composition of Sun and McDonough (1989).

Figure 13

Fig. 9. Variations in initial 87Sr/86Sr vs ϵNd(t) values for the mafic dykes studied. The field delineates the composition of Proterozoic mafic dykes within the NCC (Liu et al.2013b). The mafic dykes analysed during this study plot within the isotope-depleted mantle source field.

Figure 14

Fig. 10. Plots of zircon ages vs ϵHf(t) values for the mafic dykes studied.

Figure 15

Fig. 11. Variations in La vs La/Sm and Sm vs Sm/Yb of the mafic dykes studied.

Figure 16

Fig. 12. The correlative plots MgO vs SiO2 (a) and Cr vs Ni (b) for the mafic dykes studied.