Crust–mantle interaction induced by deep subduction of the
continental crust: geochemical and Sr–Nd isotopic evidence from
post-collisional mafic–ultramafic intrusions of the northern Dabie
complex, central China
Bor-ming Jahn
a,), Fuyuan Wu
a,b, Ching-Hua Lo
c, Chin-Ho Tsai
d aGeosciences Rennes, UniÕersite de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France´ ´
b
Department of Geology, Changchun UniÕersity of Earth Sciences, Changchun 130061, China c
Department of Geology, National Taiwan UniÕersity, 245 Choushan Rd., Taipei 10770, Taiwan d
Department of Geological and EnÕironmental Sciences, Stanford UniÕersity, Stanford, CA 94305, USA
Received 20 May 1998; revised 2 December 1998; accepted 2 December 1998
Abstract
Interaction of deeply subducted continental blocks with the upper mantle peridotite is a likely process, but it has never been demonstrated. New geochemical and isotope tracer analyses of post-collisional mafic–ultramafic rocks from the Dabie terrane in central China show that they could have been generated by melting of such metasomatized mantle as a result of
Ž .
crust–mantle interaction. Isotopic dating using different techniques Rb–Sr, Sm–Nd and Ar–Ar has established that these
Ž .
mafic–ultramafic rocks were emplaced post-tectonically in early Cretaceous f 120–130 Ma , nearly contemporaneous with the massive intrusions of granitic plutons. They did not form as part of the early Paleozoic arc complex, nor did they
Ž .
undergo UHP metamorphism at about 220 Ma. The strong enrichment of light rare earth elements REE and the highly
Ž . Ž .
negative ´NdT values about y15 to y20 for all mafic and ultramafic rocks indicate their derivation from an enriched
mantle source. Significant negative Nb anomalies observed in the spidergrams and other ‘crustal’ signatures of these rocks suggest an important role of continental material in their petrogenesis. We interpret that the singular geochemical and
Ž .
isotopic characteristics witness a post-collisional interaction between the subducted ancient crust Yangtze craton and the
Ž .
mantle peridotite asthenosphere . Partial melting of such metasomatized mantle produced the basic magmas, in response to the same thermal pulse that was responsible for the massive Cretaceous granitic intrusions and resetting of some isotopic clocks in UHP metamorphic rocks. Taking into consideration all geochemical and isotopic constraints, a tectonic model is presented with emphasis on the post-collisional crust–mantle interaction and possible heat sources required for massive Cretaceous granitic intrusions. We also advocate that the digestion of deeply subducted continental blocks in the upper mantle may represent an efficient way of crustal recycling when dealing with the problem of continental growth and destruction. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Dabie terrane; UHP terrane; Continental subduction; Mafic–ultramafic intrusion; Crust–mantle interaction; Age dating; Sr–Nd
isotopes
)
Corresponding author. Tel.: q33-99-28-60-83; fax: q33-299-28-67-72; e-mail: jahn@univ-rennes1.fr
0009-2541r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
Ž .
1. Introduction
The Dabie Mountains and the Su-Lu region in central China are known to contain the largest
distri-Ž .
bution of ultrahigh pressure metamorphic UHPM rocks in the world. They are parts of the Qinling– Dabie orogen formed by collision between the
Sino-Ž .
Korean and Yangtse cratons Fig. 1 . Coesite-bearing
Ž .
rocks mainly eclogites are widely distributed in the two terranes which are truncated by the Tanlu Fault and offset by about 500 km. The preservation of UHP minerals has inspired different models to ex-plain the genesis of extreme crustal thicknesses and the mechanisms for subsequent unroofing of the
Ž
UHPM rocks e.g., Chopin, 1984; Anderson et al., 1991; Avigad, 1992; Michard et al., 1993; Davis and
.
von Blanckenburg, 1995; Hacker et al., 1995, 1998 . In the absence of good constraints from geochemical, isotopic and age data for different lithotectonic units,
Ž
numerous tectonic models have been proposed Mat-tauer et al., 1985, 1991; Okay and Sengor, 1992; Xu
¨
et al., 1992; Yin and Nie, 1993; Li, 1994; Wang,.
1996 . It appears that the greatest controversy to date concerns the primary ages for different lithological units, the nature of eclogite protoliths, their relation with the associated ultramafic rocks and enclosing
Ž
granitic gneisses or the ‘in-situ’ vs. ‘foreign’
hy-.
pothesis , the areal extent of the UHPM rocks, and their mode of exhumation. A comprehensive study of geochemical and Nd–Sr isotopic characteristics for different types of eclogites and their associated ultra-mafic rocks from the Dabie and Su-Lu UHP terranes
Ž .
was presented by Jahn 1998 and the data were used
Ž .
to constrain 1 the petrogenesis of the coesite-bearing
Ž .
eclogites and associated ultramafic rocks, and 2 the tectonic evolution of the Qinling–Dabie collisional belt.
Subduction of continental blocks such as shown in the Dabie UHP terrane may have a direct implica-tion for crustal recycling and a likely consequence of crust–mantle interaction. We have ‘discovered’ that post-collisional mafic intrusions in the northern Dabie
Ž .
complex NDC of the Dabie terrane possess highly unusual geochemical and isotopic characteristics that may be used to argue for the effect of crust–mantle
Ž .
interaction. The purposes of this paper are: 1 to present new age information for the magmatic intru-sions using Rb–Sr, Sm–Nd and Ar–Ar isotopic
analyses on whole-rock samples and their mineral
Ž .
constituents, 2 to use geochemical and Sr–Nd iso-tope tracers to constrain the petrogenetic processes
Ž .
of these rocks, and 3 to discuss tectonic implica-tions for the Dabie collisional orogen.
2. General geology of the NDC
The general geology of the Su-Lu and Dabie UHP metamorphic terranes has been described in
numer-Ž
ous recent publications Eide, 1995; Wang et al., 1995; Cong, 1996; Hacker et al., 1996; Liou et al.,
.
1996; among others . Briefly, the Dabie terrane is
Ž .
composed of three major petrotectonic units: 1 the
Ž .
northern Dabie orthogneiss complex, 2 the central
Ž .
Dabie UHP metamorphic complex, and 3 the southern Dabie HP blueschistrgreenschist terrane. They are bounded in the south by a Triassic foreland fold-thrust belt and in the north by a greenschist facies meta-sedimentary unit, the Foziling Group, which is composed of several kilometers of quartzite
Ž
and biotite–muscovite quartz schist RGS Anhui,
.
1987 , and has been commonly interpreted as flysch deposits or as passive continental apron deposits of
Ž .
the Yangtze craton Okay et al., 1993 . The Foziling Group is equivalent to the North Huaiyang Flysch Belt used by other authors. All three petrotectonic units are intruded by Cretaceous granitoids.
Ž .
The NDC Fig. 1 consists dominantly of granitic
Ž
gneisses of TTG compositions trondhjemite–tona-lite–granodiorite; Jahn et al., 1994, 1995; Wang et
.
al., 1996 and subordinate migmatite, amphibolite, garnet granulite, marble and some conspicuous trains
Ž
of mafic–ultramafic rocks Wang and Liou, 1991; Okay et al., 1993; Wang et al., 1996; Zhang et al.,
.
1996 . Eclogitic rocks have not been undisputably identified, but UHP metamorphism has been inferred
Ž
based on some relic mineralogy Tsai and Liou,
.
1997 . The bulk of the NDC has been interpreted as
Ž
a Cretaceous extensional-magmatic complex Hacker
.
et al., 1995, 1998 . The most deformed zone is within greenschist-facies mylonites and ultra-mylonites along the Xiaotian–Mozitang detachment fault at the northern topographic limit of the Dabie
Ž .
Mountains Hacker and Wang, 1995 . The common gneisses show banded texture and are strongly
foli-() Jahn et al. r Chemical Geology 157 1999 119 – 146 121
ated, medium-grained, frequently porphyroblastic, containing enclaves of amphibolites. The banding in the gneisses plunges to the north and northwest at 30–608 angle. Migmatitic gneisses show partial melt-ing structures and the zones of meltmelt-ing also form ductile shear zones suggesting syndeformational
par-Ž .
tial melting Okay et al., 1993 . The NDC is in general characterized by amphibolite facies meta-morphism and partial melting. However, the pres-ence of mafic granulites at several localities suggests that the NDC may have reached granulite facies metamorphism that was strongly overprinted by the amphibolite facies and later thermal event when massive Cretaceous granites were emplaced. Further-more, preservation of garnet growth zoning in a
Ž
felsic granulite from Luotian NW of Yinshan, out of
. Ž .
map in Fig. 1 led Chen et al. 1998 to suggest a short residence time for the granulite at peak meta-morphism and thus a rapid tectonic uplift history.
Because of the intense deformation and thermal recrystallisation prior to and during the Cretaceous granitic intrusion, the metamorphic complex has been interpreted as a thermally overprinted subduction
Ž
complex Wang and Liou, 1991; Okay and Sengor,
¨
.
1992; Maruyama et al., 1994 , as a Paleozoic
An-Ž .
dean magmatic arc complex Wang et al., 1996 , as a
Ž
metamorphic ophiolite melange complex Xu et al.,
.
1992, 1994, 1995 , as a Cretaceous magmatic belt
Ž
formed during large-scale extension Hacker et al.,
.
1995 , or as the Sino-Korean hangingwall during Triassic subduction for the formation of the UHP
Ž .
units to the south Zhang et al., 1996 .
Ž .
The ages of the protolith s for most lithologic units are not yet determined. Our Sm–Nd isotopic analyses of granitic gneisses give TDM model ages of 1.5–1.8 Ga which provide the upper limit for the ages of their protoliths. Biotite and hornblende from the orthogneiss complex yielded Ar–Ar ages of
Ž .
120–130 Ma Hacker and Wang, 1995 and biotite– plagioclase–whole-rock Rb–Sr isochrons gave about
Ž .
115 Ma Potel and Jahn, unpublished . These ages are similar to the cooling ages of the widespread Cretaceous granitic intrusions, and were once be-lieved to reflect reheating by post-collisional
mag-Ž .
maticrextensioanl event Hacker and Wang, 1995 .
Ž
The latest zircon age studies Xue et al., 1997;
.
Hacker et al., 1998 suggest that the bulk of the NDC is a Cretaceous magmatic complex, and the
pre-Cretaceous ‘basement rocks’, represented by garnet granulite with minor marble and ultramafic rocks, are only scraps of the Yangtze craton that
Ž .
survived the deep subduction Hacker et al., 1998 . This interpretation is quite revolutionary. A further
Ž .
discussion will be presented later Section 4.4 .
Ž
Mafic and ultramafic rocks gabbros and
pyroxen-.
ites are widely distributed and form several linear
Ž .
alignments Fig. 1 . Most of them represent a post-tectonic intrusive complex, comprising more than 130 composite bodies of variable dimensions ranging
2 Ž .
from 0.2 = 0.5 to 2 = 8 km Zhang et al., 1996 . However, in the past these rocks have been variably, and erroneously, considered as ophiolite suites or
Ž
subducted Tethys oceanic lithosphere Liu and Hao,
.
1989; Xu et al., 1992; Okay, 1993; Xu et al., 1995 , or as components of the arc complex formed in early Paleozoic and prior to the continental collision that
Ž
produced the Qinling–Dabie orogen Li et al.,
.
1989a,b, 1993; Wang et al., 1996 . The intrusive mafic–ultramafic rocks are generally undeformed, show no or little sign of metamorphism, and display distinct cumulate textures and intrusive relations with
Ž .
country gneisses. Gabbros from Shacun Fig. 1 have a pegmatitic texture with grain size up to 2 cm. At
Ž .
Zhujiapu Fig. 1 , coarse-grained pyroxenite is cut by fine-grained gabbro and anorthosite dikes.
Alpine-type ultramafic blocks are relatively rare and the most representative is the Raobazhai
‘mas-Ž .
sif’ near Mozitan Fig. 1 . This ‘massif’, or better termed as tectonic slice based on unpublished Chi-nese drilling reports, consists mainly of Cr-spinel harzburgite, dunite, and lherzolitic mylonite; it is in fault contact with the surrounding migmatitic
or-Ž .
thogneisses Xu et al., 1994; Tsai and Liou, 1997 . Minor plagioclase hornblendite dikes crop out near the rim of this massif. The entire massif was defi-nitely not emplaced as intrusive body. It has been recrystallized in granulite- to amphibolite-facies
con-Ž .
ditions 700–8008C , presumably isofacial with the
Ž .
surrounding gneisses Tsai et al., 1998 . A higher pressure precursor stage is possible, but direct evi-dence for UHP is still lacking. The Raobazhai mas-sif, with its Sm–Nd metamorphic age of f 240 Ma
Ž . Ž .
and ´Nd T f y3 Li et al., 1993 , is genetically
unrelated to the rocks studied herein.
It should be noted that mafic–ultramafic intru-sions are not limited to the NDC. They also occur in
the North Huaiyang Belt to the north and in the
Ž .
southern Dabie HP terrane RGS Anhui, 1987 . In addition to the gneiss complex and mafic–ultamafic massifs, the NDC contains voluminous Cretaceous granitic intrusions. Geochemical and Sr–Nd–Pb iso-topic tracer studies indicate that these granites were
Ž .
derived from remelting of old mid-Proterozoic? continental crust, probably at a lower crustal level
ŽZhou et al., 1992, 1995b; Zhang et al., 1995; Xie et
.
al., 1996; Chen and Jahn, 1998 .
3. Analytical procedures
Four gabbro, three diorite and one olivine pyrox-enite samples from the NDC have been analysed for chemical and Sr–Nd isotopic compositions. Petro-graphic descriptions for the analyzed samples are given in Appendix A.
3.1. Elemental abundances
Whole-rock powdered samples were prepared in agate mortars in order to minimize potential contam-ination of transition metals. Mineral separates were done mainly by magnetic separation and further puri-fied by hand-picking. Major and trace element
con-Ž .
tents except REE were measured by XRF using a Philips PW1480 spectrometer in Rennes. Analytical uncertainties are "1% to 3% for major elements;
"5% for trace elements G 20 ppm and "10% for
those F 20 ppm. REE abundances were determined by the isotope dilution method using a single-collec-tor Cameca TSN-260 mass spectrometer. Uncertain-ties are "3% for La and Lu and "2% for other REE’s.
3.2. Sr–Nd isotopic analyses
The analytical procedures for isotopic analyses
Ž
are the same as reported earlier Chavagnac and
.
Jahn, 1996; Jahn et al., 1996 . Analytical precisions, Sr–Nd isotope standard and normalization values, and blank levels can be found in the footnotes of
Ž .
data tables. The decay constants l used in age
computation are: 87Rb s 0.0142 Gay1
and 147Sm s
y1 Ž .
0.00654 Ga . Model ages TDM were calculated
using the following equation assuming a linear Nd isotopic growth of the depleted mantle reservoir from ´Nds0 at 4.56 Ga to ´Nds q10 at the present: 143 144 TDMs1rl ln
½
Ž
Ndr Nd y 0.51315.
s 147 144 rŽ
Smr Nd y 0.2137.
5
sRb–Sr and Sm–Nd isochron calculations were done
Ž
using the regression programs of ISOPLOT Ludwig,
.
1990 . Input errors used in age computations are:
1 4 7
S m r1 4 4N d s 0 .2 % , 1 4 3N d r1 4 4 N d s 0.005%;87Rbr86Sr s 2%, and 87Srr86Sr s 0.005%. Analytical precisions of isotope ratio measurements
Ž .
are given as "2 standard errors 2 s , whereas them
quoted errors in age and initial isotopic ratios
repre-Ž .
sent "2 standard deviations 2 s .
3.3. Ar–Ar analyses
Mineral separation was achieved using a combina-tion of magnetic, heavy liquid, and hand-picking techniques. The 80–120 mesh fractions were se-lected for argon isotope analyses. Weighed aliquots of mineral separates were wrapped in aluminum foil packets and stacked in an aluminum canister with the
Ž .
irradiation standard LP-6 biotite Odin et al., 1982 to monitor the neutron flux. They were irradiated in the VT-C position of the THOR Reactor at Tsing-Hua
Ž .
University Taiwan for 8 h with a fast neutron flux
13 Ž 2 .
of 1.566 = 10 nr cm s . After irradiation, the samples were degassed in steps from 550 to 12008C with a 30 minrstep heating schedule, and the puri-fied gas was analyzed with a VG3600 mass spec-trometer at National Taiwan University. The concen-trations of 36Ar,37Ar,38Ar,39Ar and 40Ar were cor-rected for system blank, radioactive decay of nucle-ogenic isotopes, and minor interference reactions involving Ca, K and Cl. The detail analytical and correction techniques have been discussed by Lo and
Ž .
Lee 1994 .
The 40Arr39Ar data were plotted on apparent age spectrum and 36Arr40Ar–39Arr40Ar isotope correla-tion diagrams. The integrated dates were calculated from the sum total of the peak heights and their errors from the square root of the sum of squares of the peak height errors for all temperature steps. The
plateau dates were calculated by the same approach but utilizing only those dates on the plateau. A
Ž .
‘plateau’ is defined if: 1 there are at least four successive temperature steps with dates that fall
Ž .
within 2 s of the average, 2 the gas fraction for the plateau steps should consist of more than 50% of
39
Ž .
total Ar released, 3 the plateau steps should yield
Ž .
a linear array on the isotope correlation or isochron diagram with an acceptable goodness of fitting pa-rameters, i.e., mean square of weighted deviates
Žhereafter MSWD , 4 the. Ž . 40Arr Ar intercept value36
Ž .
obtained from the isotope correlation or isochron diagram should not be significantly different from
Ž 40 36 .
the atmospheric ratio i.e., Arr Ar s 295.5 , and
Ž .5 the plateau date and the intercept isochron dateŽ .
on the isotope correlation diagram should be
concor-Ž .
dant Lanphere and Dalrymple, 1978 . In the isotope correlation diagram, the regression line yields two intercepts. The inverse of 39Arr40Ar intercept pro-duces a so-called intercept date, whereas the inverse of the36Arr40Ar intercept indicates the composition of a non-radiogenic argon component. The cubic
Ž .
least-square fitting scheme outlined by York 1969 was employed in regressing the data. The CarK and ClrK ratios for gas from each temperature step were derived from the measured 37Ar rCa 39ArK and
38
Ar rCl 39ArK ratios according to the relationships CarK s 1.78 =37Ar rCa 39ArK and ClrK s 0.52 =
38
Ar rCl 39ArK for the samples irradiated at the THOR
Ž .
Reactor Lo and Lee, 1994 .
4. Results and discussion 4.1. Geochemical characteristics
The results of major and trace element analyses are given in Table 1. Some notable geochemical features are summarized below.
Ž .1 Fig. 2a and b shows the variation of major and trace elements as a function of mg values, which serve as a rough index of magmatic differentiation.
Ž .
The overall variations in major elements Fig. 2a are consistent with the change of mg values except for a
Ž .
high-Mg diorite BJ93-23 from Shacun and a
gab-Ž .
bro or gabbroic diorite from Zhujiapu mg s 43 .
Ž .2 BJ93-23 appears to have a composition similar to that of a high-Mg andesite or boninite formed by
Ž
wet melting of mantle peridotite e.g., Tatsumi and
.
Eggins, 1995 . However, this sample does not repre-sent an andesitic liquid, but is an amphibole cumu-late complementary to the light-colored diorite
ŽBJ93-22. Žsee Appendix A ..
Ž .3 All chondrite-normalized REE patterns are
Ž .
highly enriched in light REE’s Fig. 3 . They are completely different from those of ocean floor basalts
ŽN-MORB which are characterized by LREE deple-.
tion. Any assignment of the mafic–ultramafic rocks
Ž .
as ophiolite suites e.g., Xu et al., 1994 must be erroneous. The REE patterns of gabbros and diorites
Žexcept the high-Mg one from Shacun are compara-.
ble with those of alkali basalts and their
differenti-Ž .
ates e.g., trachyandesites , but there are sufficient differences between them in terms of total alkalis, TiO , P O2 2 5 and Nb abundances. The pyroxenite
ŽBJ95-03 and fine-grained gabbro BJ95-04 from. Ž .
Ž .
Zhujiapu Table 1; Fig. 3 appear to have a genetic relationship, with pyroxenite as Cpx–Opx cumulate
Ž
from a more primitive gabbroic magma not
sam-. Ž .
pled whereas the fine-grained gabbro or diorite
Ž
represents a differentiated liquid very low Ni and Cr contents of 8 and 6 ppm, respectively; data
con-.
firmed by duplicate analyses from the same magma
Ž . Ž
but was enriched in Al O2 3 f20% , Zr f500
. Ž . Ž .
ppm , Sr f 1500 ppm and Ba f 2400 ppm . In fact, sample BJ95-04 is characterized by high modal abundance of plagioclase and accessory apatite and
Notes to Table 1:
Ž .1 Major and trace elements analyzed by XRF Philips PW1480 spectrometer in Rennes.Ž .
Ž .2 Uncertainties XRF : "1% to 3% for major elements; "5% for trace elements G 20 ppm and "10% for those F 20 ppm.Ž . Ž .3 Uncertainties ID : "3% for La and Lu, and "2% for other REE.Ž .
Ž .4 mg value s molecular proportion of MgOr MgO q FeO , assuming 90% of total iron oxides as FeO. mg value s MgO r MgO q 0.505Ž . w x w x ŽwFeO q 0.9 Fe Ox w 2 3x.4, if FeO and Fe O are reported separately.2 3
Table 1
Chemical compositions of mafic–ultramafic bodies—the NDC
Sample no. BJ93-10 BJ93-11 BJ93-21 BJ93-22 BJ93-23 BJ95-03 BJ95-04 95DB-13P 921-2 9212-1
Ž .
Analysis no. 12386 12387 12394 12395 12396 12862 12863 13744 Wang et al., 1996
Locality Shacun Shacun Shacun Shacun Shacun Zhujiapu Zhujiapu Jiaoziyan Raobazhai Raobazhai Rock type Gabbro Diorite Gabbro Q. diorite Diorite Px’enite Gabbro Gabbro Gabbro Px’enite
SiO2 47.38 60.41 48.07 62.19 54.15 47.20 50.72 50.10 47.55 46.78 Al O2 3 10.84 15.35 6.95 15.65 9.76 3.26 20.36 17.30 14.03 3.25 Fe O2 3 11.01 6.25 11.88 5.57 8.80 12.08 8.71 8.52 10.80 10.37 MnO 0.17 0.10 0.19 0.09 0.14 0.21 0.07 0.14 0.20 0.16 MgO 11.08 3.36 15.94 2.37 11.97 22.71 2.96 6.47 9.50 18.24 CaO 13.57 4.96 11.54 4.51 7.18 12.39 6.44 9.65 9.63 17.52 Na O2 1.84 3.58 1.20 3.67 2.08 0.36 5.23 3.20 2.74 0.25 K O2 0.93 3.20 0.94 3.33 1.52 0.17 1.75 1.23 1.48 0.02 TiO2 1.46 0.80 0.78 0.72 0.94 0.34 1.07 1.12 0.71 0.92 P O2 5 0.18 0.33 0.13 0.26 0.30 0.04 0.79 0.29 0.24 0.01 LOI 0.97 0.78 1.75 0.66 2.16 0.46 0.93 0.45 3.37 2.41 Total 99.43 99.12 99.37 99.02 99.00 99.22 99.03 98.47 100.25 99.93 mg 69 54 75 48 75 81 43 63 66 79 ( ) Trace elements ppm , by XRF Nb 4.8 10.8 3.5 10.2 8.5 1 9 7.68 2 0.95 Zr 84 226 90 205 152 26 509 122 61 22 Y 32 24 20 23 20 11 28 18 22 5.6 Sr 518 671 295 658 452 87 1493 923 727 224 Rb 20 77 27 91 40 3 52 23 31 1.2 Co 44 17 72 15 55 90 26 43 84 V 317 132 199 113 122 146 129 55 286 Ni 87 27 271 12 369 399 8 102 449 Cr 586 110 636 29 969 1224 6 345 1470 Ba 448 1514 492 1415 855 77 2370 636 1100 149 Ga 14 19 10 19 14 5 25 Cu 39 31 85 16 23 87 139 Zn 78 69 86 62 94 77 75 Th 2 10 3 10 7 1 2 0.8 0.69 0.06 Pb 9 15 8 20 15 - 1 10 6.59 ( ) By ID ppm La 16.15 48.35 16.06 43.37 37.33 3.91 61.10 26.74 17.90 1.12 Ce 38.50 94.01 37.28 85.96 76.33 10.56 123.31 56.23 41.40 5.14 Nd 26.53 38.82 19.75 35.16 34.98 8.13 55.54 27.05 22.30 5.32 Sm 6.63 6.66 4.53 6.15 6.49 2.20 9.27 5.59 4.84 1.70 Eu 1.951 1.747 1.271 1.572 1.675 0.644 2.586 2.012 1.89 0.73 Gd 6.69 5.26 4.24 4.92 4.93 2.33 6.95 5.662 5.42 1.99 Tb 0.701 0.91 0.33 Dy 5.42 3.96 3.32 3.62 3.58 2.00 4.72 3.829 Ho 0.701 1.17 0.37 Er 2.88 2.18 1.64 1.98 1.79 1.07 2.29 1.816 Tm 0.45 0.12 Yb 2.26 1.98 1.35 1.80 1.41 0.93 1.88 1.51 2.64 0.63 Lu 0.331 0.307 0.194 0.278 0.213 0.136 0.277 0.224 0.34 0.08 ŽLarYb.n 4.7 16.1 7.9 16.0 17.5 2.8 21.5 11.7 4.5 1.2 SmrNd 0.250 0.172 0.229 0.175 0.186 0.271 0.167 0.207 0.217 0.320 BarNb 93 140 141 139 101 77 263 83 550 157 LarNb 3.4 4.5 4.6 4.3 4.4 3.9 6.8 3.5 9.0 1.2
Ž .
Fig. 2. Geochemical variation diagrams for major and trace elements as a function of mg values as a rough index of differentiation . mg
Ž .
value is the molecular proportion of MgOr MgO q FeO , assuming 90% of total iron as ferrous iron. Note that two rocks from Raobazhai fall outside of the trends in most cases. R s Raobazhai, Z s Zhujiapu, S s Shacun, J s Jiaoziyan.
sphene, thus, leading to low mg value, low Ni and Cr, but high Al, Ti, Sr and Ba contents.
Ž .4 In the primitive mantle PM normalized geo-Ž .
Ž .
chemical spidergrams Fig. 4 all the rocks show
very distinctive negative anomalies in Nb, P and Ti, and positive anomalies in Pb. Negative Nb anomaly is most characteristic of subduction zone volcanic rocks or typical continental crust. Since a subduction
Fig. 3. Chondrite-normalized REE patterns for mafic–ultramafic rocks of the NDC. All gabbros and diorites are highly enriched in light REE’s. The Zhujiapu pyroxenite pattern is consistent with that formed by accumulation of pyroxenes from a liquid of
w Ž
LREE-enriched pattern. Chondrite values 1.2= Masuda et al.,
. x
1973 ; in ppm used for normalisation are: La s 0.315, Ces 0.813, Nd s 0.595, Sm s 0.193, Eu s 0.0722, Gd s 0.259, Dy s 0.325, Er s 0.213, Ybs 0.208, Lu s 0.0323.
is not considered plausible in the post-collisional tectonic setting in the Dabie terrane, the ‘continental’
Fig. 4. PM normalized spidergrams of the mafic–ultramafic rocks. Conspicuous negative anomalies in Nb, P, and Ti and positive Pb
Ž .
anomalies are observed in all samples. The PM values in ppm
Ž .
used are from Sun and McDonough 1989 : Rbs 0.635; Ba s 6.99; Th s 0.056; Us 0.021; K s 249; Nbs 0.713; La s 0.687; Ces1.775; Sr s 21.1; P s96; Nd s1.354; Zr s11.2; Sm s 0.444; Eu s 0.168; Ti s1300; Gd s 0.596; Dy s 0.737; Ys 4.55; Er s 0.48; Ybs 0.493; Lu s 0.077.
signature of mantle-derived magmas must have a special implication.
Ž .5 LarNb ratios in gabbros, diorites and pyrox-enite are relatively uniform about 4 and so are
Ž .
BarNb ratios about 100 Fig. 5; Table 1 . These values are substantially different from those of most intraplate volcanic rocks including N-MORB, OIB, alkali basalts and kimberlites which have LarNb ratios of 2.5 to 0.5 and much smaller BarNb ratios
Ž .
of 20 to 1 Fig. 5 . The data suggest a role of
Ž
continental rocks granitoids, granulites, sediments,
.
etc. in the magma genesis of the mafic–ultramafic suite.
In summary, the geochemical characteristics of these mantle-derived magmas and their differentiates are highly unusual in that they contain a clear conti-nental signature which is unrelated to subduction
Fig. 5. BarNb vs. LarNb plot showing that the mafic–ultramafic rocks are characterized by high BarNb and LarNb ratios, falling in the fields of arc volcanics and Archean granulites from eastern
Ž .
Hebei data from Jahn and Zhang, 1984 . Magmatic differentia-tion tends to increase both ratios. The granulite data are used to infer the composition of the middle to lower continental crust, but not to imply a connection with the Sino-Korean craton. Data
Ž .
sources for other fields: PM Sun and McDonough, 1989 , CC
Žcontinental crust average Taylor and McLennan, 1985; Condie,. Ž
. Ž .
1993 , Clastic sediment average Condie, 1993 , MORB, OIB and
Ž .
Table 2
Whole-rock Rb–Sr and Sm–Nd isotopic compositions of mafic–ultramafic rocks from the NDC
87 87
w x w x
Sample Analysis Rock type Locality Rb Sr Rbr Srr "2 sm ISr
86 86
Ž . Ž . Ž .
no. no. ppm ppm Sr Sr 120 Ma
( )A North Dabie complex
BJ93-10 12386 gabbro Siling 19.7 515.5 0.111 0.707640 8 0.70745
BJ93-11 12387 diorite Shacun 79.1 675.3 0.339 0.708141 7 0.70756
BJ93-21 12394 gabbro Shacun 24.5 278.0 0.255 0.707885 7 0.70745
Ž .
BJ93-22 12395 diorite light Shacun 85.8 662.4 0.375 0.708556 7 0.70792
Ž .
BJ93-23 12396 diorite dark Shacun 39.1 429.7 0.263 0.708121 7 0.70767
BJ95-03 12862 pyroxenite Zhujiapu 3.02 84.1 0.104 0.707689 6 0.70751
Žduplicate. 3.22 85.9 0.108 0.707709 5 0.70752
BJ95-04 12863 gabbro Zhujiapu 51.3 1559 0.095 0.708171 6 0.70801
95DP-13P 13744 gabbro Jiaoziyan 23.0 923.3 0.072 0.706839 6 0.70672
Žduplicate. 0.706862 6
DZh-1a diorite Zhujiapu
DZh-1b diorite Zhujiapu
( )B Noth Huaiyang Flysch Belt
9101 diorite Shanqilihe 96.9 1147 0.245 0.709180 10 0.70876
9105 gabbro Wangjiachong 38.9 731.3 0.154 0.709700 20 0.70944
9108 gabbro Wangjiachong 6.17 1182 0.015 0.706790 20 0.70676
Ž .1 143Ndr144Nd ratios have been corrected for mass fractionation relative to146Ndr144Nd s 0.7219 and are reported relative to the La Jolla Nd standard s 0.511860 or Ames Nd standard s 0.511962.
Ž .2 87Srr Sr ratios have been corrected for mass fractionation relative to86 86Srr Sr s 0.1194 and are reported relative to the NBS-987 Sr88 standard s 0.710250.
Ž .3 CHUR chondritic uniform reservoir :Ž . 147Smr144Nd s 0.1967;143Ndr144Nd s 0.512638.
Ž .4 Used in model age calculation, DM depleted mantle :Ž . 147Smr144Nd s 0.2137;143Ndr144Nd s 0.51315.
Ž .5 Blanks: Rb s 30 pg, Sr s 100 pg, Sm s 37 pg, Nd s 100 pg.
because this process was absent during the Creta-ceous. It remains to be determined whether it means
Ž .1 crustal contamination during magma
differentia-Ž .
tion, 2 partial melting of mantle peridotites highly metasomatized by interaction with deeply subducted
Ž .
continental crust during the Triassic collision, or 3 a combination of both processes.
4.2. Mineral ages
4.2.1. Rb–Sr isotope systematics
Because the mafic and ultramafic rocks are not metamorphosed, we opted to use mineral isochron methods for dating the intrusive or cooling events. The results of isotopic analyses for whole-rock sam-ples and constituent minerals are given in Tables 2 and 3, respectively. In Table 2, we include published
Ž
data of three Cretaceous bodies two gabbros and a
.
diorite emplaced in the North Huaiyang Belt. These rocks are isotopically identical to those of the present study, thus a similar petrogenesis is implied. A
gab-Ž .
bro from Shacun BJ93-21 yielded a Rb–Sr isochron
Ž . Ž
age of 123 " 6 2 s Ma ISOPLOT, Model 3
solu-. Ž .
tion , with I s 0.70738 " 1 Fig. 6a . The isochronSr
age is evidently controlled by biotite, so it is consid-ered as a biotite cooling age. Fine-grained gabbroic
Ž .
dike BJ95-04 from Zhujiapu gave a biotite–WR– plagioclase Rb–Sr isochron age of 118 " 2 Ma, with
I s 0.70801 " 1. Again, this is a biotite cooling ageSr
for the dike intrusion. The age of the coarse-grained
Ž .
intruded pyroxenite BJ95-03 was not obtained by the Rb–Sr analyses due to slight open system behav-ior of the analyzed phases. However, the pyroxenite possesses an initial 87Srr86Sr ratio of about 0.7075, distinctly lower than the ‘geochemically’ cogenetic
Ž .
gabbroic dike Fig. 6b . This may suggest that crustal contamination has exerted variable effects on the two
147 143 wSmx wNdx Smr Ndr "2 sm ´NdŽ .0 ´NdŽT. f Smr TDM Reference 144 144 Žppm. Žppm. Nd Nd Ž120 Ma. Nd ŽGa. 6.67 26.85 0.1503 0.511682 5 y18.6 y17.9 y0.24 3.50 This study 8.49 49.65 0.1034 0.511588 5 y20.5 y19.1 y0.47 2.15 This study 4.61 20.37 0.1368 0.511809 6 y16.2 y15.3 y0.30 2.64 This study 6.14 35.16 0.1056 0.511618 8 y19.9 y18.5 y0.46 2.15 This study 6.62 35.85 0.1116 0.511624 6 y19.8 y18.5 y0.43 2.27 This study 2.19 8.01 0.1650 0.511786 6 y16.6 y16.1 y0.16 4.22 This study 2.24 8.24 0.1644 0.511796 6 y16.4 y15.9 y0.16 4.14 This study 9.52 57.05 0.1009 0.511781 6 y16.7 y15.3 y0.49 1.84 This study 5.59 27.05 0.1249 0.512066 4 y11.2 y10.1 y0.37 1.86 This study 5.84 28.06 0.1258 0.512072 6 y11.0 y10.0 y0.36 1.86 This study Ž . 7.85 31.37 0.1512 0.511757 19 y17.2 y16.5 y0.23 3.37 Li et al. 1989a,b Ž . 4.98 22.78 0.1322 0.511710 19 y18.1 y17.1 y0.33 2.68 Li et al. 1989a,b Ž .
9.76 58.41 0.1010 0.511695 6 y18.4 y16.9 y0.49 1.96 Zhou et al. 1995a
Ž .
6.56 41.16 0.0964 0.511706 10 y18.2 y16.6 y0.51 1.87 Zhou et al. 1995a
Ž .
7.53 36.41 0.1250 0.512074 6 y11.0 y9.9 y0.36 1.84 Zhou et al. 1995a
rocks, or they were derived from different sources, so their apparent cogenetic relationship was not sig-nificant. Overall, the whole-rock data points are seen
Ž .
scattered in the isochron diagram Fig. 6b , but all of them have a rather small range of initial 87Srr86Sr
Ž .
ratios from 0.7074 to 0.7080 Table 2 .
4.2.2. Sm–Nd mineral isochrons
Fig. 7 shows the Sm–Nd data points for WR and mineral separates. Like in the Rb–Sr systems, the WR data are scattered and no isochron relationship is discernible. However, mineral analyses of pyroxenite
ŽBJ95-03 yielded an isochron age of 127 " 70 2 s. Ž .
Ž .
Ma, with ´Nd T s y16.1 " 0.5. The large
uncer-tainty is due to the very small range of143Ndr144Nd ratios, even though they are well aligned and have a
Ž .
very small MSWD 0.02 . Our data appear to be in conflict with a Sm–Nd mineral isochron age of
Ž .
230 " 44 Ma reported by Li et al. 1989a , who interpreted their age to represent a ‘syncollisional’
magmatic event induced by subduction of the conti-nental crust. We shall discuss this point later.
4.2.3. Jiaoziyan Gabbro
The Jiaoziyan Gabbro is the largest Cretaceous
Ž . Ž .
mafic intrusion f 10 = 3 km in the NDC Fig. 1 .
Ž .
A gabbronorite sample 95-DB-13P was collected from an inner part of the intrusion and far away from the contact with the large Zhubuyuan granitic pluton. The sample is massive and medium-grained, and is composed of 40–45 modal % plagioclase, 30–35% augite, 10–15% orthopyroxene, about 5% biotite, and minor ilmenite, magnetite, and trace apatite. Petrographic description is given in Appendix A.
This sample was subjected to a detailed isotopic study mainly because a surprisingly ‘old’ Sm–Nd
Ž
mineral isochron age of 238 " 28 Ma or 240 " 48 Ma, recalculated using ISOPLOT with the same
.
input errors as for the present work was recently
Ž . Ž .
Table 3
Mineral Sr–Nd isotopic data of the gabbro–diorite–pyroxenite suite from the NDC
87 87
w x w x
Sample no. Rock type Phase Locality Rb Sr Rbr Srr "2 sm ISr
86 86 Žppm. Žppm. Sr Sr Ž120 Ma. BJ93-10 gabbro WR Siling 19.71 515.53 0.111 0.707640 8 0.70745 Cpx 3.77 75.56 0.144 0.707704 7 0.70746 Hb 3.90 426.59 0.026 0.707429 7 0.70738 Plag 75.40 1663.2 0.131 0.707800 6 0.70758 Ž . Pl dupl. 67.18 1675.1 0.116 0.707746 8 0.70755 BJ93-21 gabbro WR Shacun 24.54 278.0 0.255 0.707885 7 0.70745 Bi 194.7 111.1 5.077 0.716285 10 0.70763 Cpx 2.30 82.59 0.0806 0.707504 8 0.70737 Plag 9.30 1596.5 0.0169 0.707389 6 0.70736 BJ93-22 diorite WR Shacun 85.83 662.44 0.375 0.708556 7 0.70792 Bi Plag BJ93-23 diorite WR Shacun 39.09 429.69 0.263 0.708121 7 0.70767 Hb 9.61 75.45 0.369 0.708241 8 0.70761 Plag BJ95-03 pyroxenite WR Zhujiapu 3.02 84.11 0.1037 0.707689 6 0.70751 Cpx 1.79 100.86 0.051 0.707691 8 0.70760 Hb 5.60 260.88 0.062 0.707650 7 0.70754 Plag 3.20 543.55 0.017 0.707837 7 0.70781 BJ95-04 gabbro WR Zhujiapu 51.32 1559.1 0.0953 0.708171 6 0.70801 Bi 265.19 62.92 12.219 0.728461 9 0.70762 Plag 8.20 2160.4 0.011 0.708025 7 0.70801 95DP-13P gabbro WR Jiaoziyan 22.97 923.28 0.072 0.706839 6 0.70672 Žduplicate. 0.706862 6 Cpx 2.58 64.52 0.116 0.707099 7 0.70690 Žduplicate. Opx 1.98 10.2 0.56 0.707980 6 0.70702 Žduplicate. Plag 4.56 1668.7 0.0079 0.706804 5 0.70679 Žduplicate. Bio 255.02 29.51 25.097 0.746304 6 0.70350 Žduplicate.
Ž .1 143Ndr144Nd ratios was corrected for mass fractionation relative to146Ndr144Nd s 0.7219 and are reported relative to the La Jolla Nd standard s 0.511860 or Ames Nd standard s 0.511962.
Ž .2 87Srr Sr ratios have been corrected for mass fractionation relative to86 86Srr Sr s 0.1194 and are reported relative to the NBS-987 Sr88 standard s 0.710250.
Ž .3 CHUR chondritic uniform reservoir :Ž . 147Smr144Nd s 0.1967;143Ndr144Nd s 0.512638.
Ž .4 Used in model age calculation, DM depleted mantle :Ž . 147Smr144Nd s 0.2137;143Ndr144Nd s 0.51315.
Ž .5 Blanks: Rb s 30 pg, Sr s 100 pg, Sm s 37 pg, Nd s 100 pg; 45 pg.
they interpreted this age as the time of syn-colli-sional intrusion and all Rb–Sr ages as the result of isotopic resetting by the strong Cretaceous thermal event when voluminous granitic plutons were in-truded.
Our Rb–Sr analyses gave a five-point internal isochron age of 111 " 4 Ma with I s 0.7079 " 3Sr ŽFig. 8a . This is interpreted as the time when the.
gabbro cooled below the blocking temperature of
Ž .
biotite f 3008C . A rough estimate of cooling rate for such a small gabbroic pluton, assuming
reason-Ž 3.
able magma density 3000 kgrm , thermal
expan-Ž y5 . Ž y6 2 .
sion 5 = 10 rdeg , diffusivity 10 m rs ,
vis-Ž 3 . Ž
cosity 10 Pars , pluton dimension 1.5 km from
.
center , and temperature difference between magma
Ž .
and the surrounding DT s 5008C , suggests that the pluton, with convective heat dissipation, could cool down to 3008C within 1 Ma. In other words, the
147 143 wSmx wNdx Smr Ndr "2 sm ´NdŽ .0 ´NdŽT. f Smr TDM 144 144 Žppm. Žppm. Nd Nd Ž120 Ma. Nd ŽMa. 6.67 26.85 0.1503 0.511682 5 y18.6 y17.9 y0.24 3500 4.61 20.37 0.1368 0.511809 6 y16.2 y15.3 y0.30 2643 6.14 35.16 0.1056 0.511618 8 y19.9 y18.5 y0.46 2152 6.62 35.85 0.1116 0.511624 6 y19.8 y18.5 y0.43 2268 2.19 8.01 0.1650 0.511786 6 y16.6 y16.1 y0.16 4224 2.95 10.47 0.1701 0.511790 6 y16.5 y16.1 y0.14 4697 6.29 24.71 0.1540 0.511780 6 y16.7 y16.1 y0.22 3469 4.12 24.63 0.1010 0.511733 6 y17.7 y16.2 y0.49 1911 9.52 57.05 0.1009 0.511781 6 y16.7 y15.3 y0.49 1845 5.59 27.05 0.1249 0.512066 4 y11.2 y10.1 y0.37 1855 5.839 28.057 0.1258 0.512072 6 y11.0 y10.0 y0.36 1864 14.45 55.26 0.1581 0.512081 4 y10.9 y10.3 y0.20 2912 14.481 53.934 0.1623 0.512075 5 y11.0 y10.5 y0.17 3165 0.92 3.61 0.1541 0.512040 18 y11.7 y11.0 y0.22 2822 0.938 3.766 0.1506 0.512050 6 y11.5 y10.8 y0.23 2643 0.85 7.22 0.0715 0.511991 6 y12.6 y10.7 y0.64 1241 0.886 7.668 0.0698 0.511979 5 y12.9 y10.9 y0.65 1239 0.29 1.82 0.0966 0.512043 9 y11.6 y10.1 y0.51 1439 0.586 3.547 0.0999 0.512040 5 y11.7 y10.2 y0.49 1484
intrusive ages of Jiaoziyan and the smaller gabbroic and dioritic bodies would be at most a few Ma older than the biotite ages.
The inset of Fig. 8a shows that individual phases were not in isotopic equilibrium at the time of 110–120 Ma. A visual estimate of the ‘initial ratios’
Ž
at 111 Ma indicates that ISr values s intercepts of
.
the lines parallel to the biotite isochron increase in the order from WR–Plag–Bio–Cpx–Opx. As shown
Ž .
in Table 3, the calculated ISr 120 Ma vary from 0.70672 to 0.70702, except for the biotite whose ISr
is extremely sensitive to age correction due to its
Ž .
very high RbrSr ratio f 25 . The difference of 0.00030 is 50 times of the analytical precision, or 15 times of the ‘accuracy’ derived from our long-term duplicate analyses of isotope standards, hence it is considered significant, indicating isotope non-equi-librium. The fact that the WR data point does not fall
Ž .
Fig. 6. a Rb–Sr mineral isochrons for gabbros from Shacun
ŽBJ95-21. and Zhujiapu ŽBJ95-04 ,. Ž .b WR Žwhole-rock. and mineral Rb–Sr data for other samples. Each set of symbols represents WR and minerals from the same sample. Note that the WR data are highly scattered, indicating heterogeneity of their initial ratios. The mineral data for BJ93-03 and BJ93-10 do not form isochrons, probably due to post-magmatic alteration.
within the polygon formed by the other 4 phases
Ž .
suggests that some other phase s not analyzed must be responsible for balancing the WR isotopic compo-sition.
Sm–Nd isotope analyses also failed to produce
Ž .
any internal isochron Fig. 8b . We are confident that the scatter was not due to analytical errors as each data point was confirmed by duplicate analysis. The slight shift between first and duplicate runs can be ascribed to some heterogeneity of the mineral com-positions, because in our experiments only coarse mineral grains, without pulverization, were put di-rectly into dissolution. Due to the variable contents
of minute inclusions, different aliquots may have slightly different compositions. We note here once again that, as in the Rb–Sr system, the WR data point does not plot within the polygon of the four constituent phases. Evidently, some unanalyzed ac-cessory minerals must be present to account for the isotope mass balance.
In any case, both the Rb–Sr and Sm–Nd data indicate that the constituent minerals of the Jiaoziyan Gabbro are not in isotopic equilibrium. To this point, we do not understand the isochron relationship
re-Ž .
ported by previous workers e.g., Chen et al., 1997 . Interpretation of this disequilibrated mineral assem-blage is not easy. Concomittant crustal contamina-tion during magmatic differentiacontamina-tion, similar to the
Ž
AFC process assimilation and fractional
crystallisa-. Ž . Ž .
tion of Taylor 1980 and DePaolo 1981 , may result in isotopic difference between the early and late precipitated phases if assimilation continues. The crystallisation of Cpx, Opx and Plag was probably near-contemporaneous, only biotite can be shown petrographically as a late phase. At 120 Ma, the sequence of increase in initial 143Ndr144Nd ratio
ŽINd. is from Plag f Opx to Cpx to Bio Fig. 8b ;Ž .
and this could be interpreted by a hypothesis of
Ž .
upper crustal contamination higher INd in a liquid derived from a source dominated by lower crustal
Ž . Ž
component lower INd as we advocate below
Sec-Fig. 7. Sm–Nd isochron diagram for the mafic–ultramafic rocks. WR data are highly scattered, indicating heterogeneity of their
Ž .
initial ratios. Mineral data of Zhujiapu pyroxenite BJ93-03 fall on a 125 Ma reference isochron. The variation of isotope ratios is
Ž
too small to obtain an age of satisfactory precision calculated
.
Ž . Ž .
Fig. 8. a Rb–Sr isochron diagram for the Jiaoziyan gabbro. WR and constituent phases Opx, Cpx, Plag do not form a linear array with
Ž .
biotite, suggesting that these minerals were not in isotope equilibrium. b Sm–Nd isotopic data showing the absence of isochron relationship between WR and constituent minerals. The data scatter is confirmed by duplicate analyses.
.
tion 4.3 . However, since the upper crust has a higher 87Srr86Sr ratio, this does not explain the
87 86
Ž
lower Srr Sr ratio observed in Plag inset of Fig.
.
8a .
Alternatively, the Sr isotopic disequilibrium might be explained by post-magmatic alteration effect. Ex-cluding the biotite data, whose ISr value strongly
Ž
depends on the assigned age, the three phases Cpx,
.
Opx and Plag appear to show their relative
suscepti-bility toward crustal contamination as a function of their Sr concentrations. Indeed, the sequence of
in-Ž . Ž .
creasing ISr Fig. 8a is from Plag Sr s 1670 ppm
Ž . Ž .
to Cpx 65 ppm to Opx 10 ppm .
We conclude that during the intrusion of the Jiaoziyan Gabbro an AFC process was probably in effect, causing the precipitated phases not in isotopic equilibrium with each other. Consequently, no min-eral Sm–Nd isochron could have been established.
Alteration effect on Nd isotope composition is
prob-Ž
ably invisible on this little altered gabbro see
Ap-.
pendix A for petrography , but it may be significant on the Sr isotope compositions.
4.2.4. Ar–Ar plateau ages
A whole-rock sample of anorthosite dike, a horn-blende and a plagioclase from a gabbro of Zhujiapu massif were dated by the Ar–Ar method. The analyt-ical data are presented in Table 4 and the age plateaux are displayed in Fig. 9. The anothositic dike
Ž .
has an age of 116 " 3 Ma Fig. 9a , which is
identi-Ž
cal to the plagioclase age of gabbro 118 " 3 Ma;
.
Fig. 9c . These two ages are indistinguishable from the biotite–WR–plagioclase Rb–Sr isochron age
Ž118 " 2 Ma , and are also consistent with the simi-.
Ž .
lar blocking temperatures f 3008C for plagioclase Ar–Ar and biotite Rb–Sr systems. Hornblende of the
Ž .
same gabbro sample gave 131 " 3 Ma Fig. 9b ; this is explained by the higher blocking temperature of
f500–5508C. We interpret the age as the time close
to magmatic intrusion.
Ž .
Hacker and Wang 1995 dated hornblendes by
Ž
Ar–Ar from a diorite near Luzhen about 20 km NW
. Ž
of Tongcheng and a gabbro about 20 km WNW of
.
Tongcheng , both in the NE corner of the Dabie terrane. The diorite gave a plateau age of 134 " 1
Ž2 s Ma, whereas the gabbro yielded 130 " 1 Ma..
4.2.5. Conclusion from the age studies
In addition to our age data outlined above, Hacker
Ž .
et al. 1998 reported two zircon ages of 129 " 2 Ma
Žby SHRIMP and 125 " 2 Ma by TIMS for a. Ž .
gabbro from a locality near Mozitan. This is consid-ered to be the best age determination for the gab-broic intrusions in the NDC. We therefore conclude from all the geochronological information that, re-gardless of their occurrences as dike, stock, or plu-ton, most of the gabbro, diorite and pyroxenite in the NDC as well as in the North Huaiyang Flysch Belt were emplaced in a short time span from 130 to 115 Ma. This period coincides with the massive intrusion
Ž
of granitic rocks Chen et al., 1991; Zhou et al.,
.
1992; Chen et al., 1995 . These mafic–ultramafic intrusive rocks seem to share the same Sr–Nd
iso-Ž
topic characteristics with the granites to be
dis-.
cussed below . We relate these characteristics to the effect of the Triassic continental collision and
subse-Ž .
quent crust–mantle interaction mixing of sources .
4.3. Whole-rock Nd–Sr isotopic characteristics and genesis of highly negatiÕe ´N d mantle-deriÕed basic magmas
The results of whole-rock Rb–Sr and Sm–Nd isotopic analyses, together with some published data
Notes to Table 4:
Ž .
J-value: weighted mean of three fusions of irradiation standard LP-6 biotite, having a K–Ar age of 127.7 " 1.4 Ma Odin et al., 1982 . Ž .
T 8C s temperature with uncertainty of "28C.
The date is obtained by using the following equations:
1 40Ar) Date s ln 1 q J39 ,
ž
/
l ArK and 40 40 39 36 39 36 37 37 39 40Ar)
w
Arr Arx
y295.5w
Arr Arx
q295.5w
Arr Arx
w
Arr Arx
Arm m Ca m
s y
39 39 37 37 39 39
ArK 1 y
w
Arr Arx
Caw
Arr Arx
m Ar Kwx wx Ž
where Caand Ksisotope ratios of argon extracted from irradiated calcium and potassium salts values cited in the paper of Lo and Lee, . wx
1994 and msisotope ratio of argon extracted from irradiated unknown.
Ž . y1 0 y1 y10 y1
Date Ma s the date calculated using the following decay constants: l s 0.581 = 10´ year ; l s 4.962 = 10b year ; l s 5.543
= 10y1 0 yeary1;40
KrK 0.01167 at.%.
Uncertainty for40Ar) and39ArK volumes are "5%.
Cum.39Ar s cumulative fractions of39ArK and40Ar) released in each step.
The quoted error is one standard deviation and does not include the error in the J-value, the standard error, or the error in the interference corrections.
Integrated date s the date and error calculated from the sum total gas from all steps; the error includes the error in J-value.
Plateau date s the data and error calculated from the sum total gas from those steps, the ages of which fall within 2 S.D. of each other; the error includes the error in J-value.
on similar rocks from the North Huaiyang Belt, are given in Table 2. We underline the characteristic and
Ž .
very spectacular highly negative ´Nd T values for these mantle-derived rocks. Depleted mantle model
Ž .
ages TDM for the mafic–ultramafic rocks range
Ž .
from 4.2 to 1.8 Ga Table 2 . TDM values calculated
Ž147 144 .
from high SmrNd rocks Smr Nd s 0.16 of-ten gave spurious and insignificant age information
Table 4
40
Arr39Ar age data for anorthosite and gabbro samples of the NDC
39 36 39 37 39 38 39 40 39 40 39
Ž . Ž . Ž .
T 8C cum. ArK Atmos. % Arr Ar Arr Ar Arr Ar Arr Ar Arr Ar Date Ma
DB49C whole rock, Anorthosite dyke, Zhujiapu, Dabie Mt.
550 0.051 50.129 0.2061E q 00 0.8193E q 02 0.5168E y 01 0.1092E q 03 0.5298E q 03 147.2 " 6.0 650 0.200 7.541 0.2287E y 01 0.3686E q 02 0.1776E y 01 0.5281E q 02 0.2309E q 04 128.7 " 3.5 750 0.313 2.406 0.8486E y 02 0.1871E q 02 0.1531E y 01 0.4567E q 02 0.5381E q 04 116.5 " 5.3 850 0.408 0.636 0.8433E y 02 0.2934E q 02 0.1471E y 01 0.4426E q 02 0.5249E q 04 115.8 " 5.9 950 0.499 0.058 0.9534E y 02 0.3705E q 02 0.1709E y 01 0.4369E q 02 0.4583E q 04 115.5 " 4.4 1000 0.554 5.062 0.1827E y 01 0.4067E q 02 0.1933E y 01 0.4613E q 02 0.2525E q 04 116.1 " 6.6 1100 0.688 8.939 0.2067E y 01 0.2171E q 02 0.1889E y 01 0.5005E q 02 0.2422E q 04 119.2 " 3.5 1200 1.000 14.429 0.2667E y 01 0.5123E q 01 0.1832E y 01 0.5198E q 02 0.1949E q 04 115.2 " 2.9 Sample mass s 0.0800 g
J-value s 0.0014789 " 0.000037193
Integrated date s 119.7 " 3.1 Ma
Ž .
Plateau date s 116.2 " 3.0 Ma 750–12008C
DB49B hornblende, Gabbro, Zhujiapu, Dabie Mt.
550 0.161 29.096 0.7528E y 01 0.8248E q 01 0.3118E y 01 0.7435E q 02 0.9876E q 03 136.1 " 3.7 600 0.350 3.716 0.9664E y 02 0.1085E q 02 0.1711E y 01 0.5487E q 02 0.5678E q 04 136.6 " 3.9 700 0.553 0.001 0.6875E y 02 0.3281E q 02 0.1691E y 01 0.5094E q 02 0.7409E q 04 133.7 " 4.6 800 0.638 0.449 0.1384E y 01 0.5150E q 02 0.2274E y 01 0.4662E q 02 0.3368E q 04 123.7 " 6.1 900 0.738 0.420 0.1559E y 01 0.5864E q 02 0.2008E y 01 0.4516E q 02 0.2896E q 04 120.6 " 4.8 1000 0.796 2.237 0.2558E y 01 0.8610E q 02 0.2897E y 01 0.4791E q 02 0.1873E q 04 127.7 " 7.6 1080 0.840 15.380 0.4550E y 01 0.6432E q 02 0.3673E y 01 0.5594E q 02 0.1229E q 04 127.2 " 6.6 1160 0.935 19.414 0.4638E y 01 0.2840E q 02 0.3348E y 01 0.5960E q 02 0.1285E q 04 125.9 " 4.5 1230 1.000 29.637 0.6236E y 01 0.4196E q 04 0.3843E y 01 0.6220E q 02 0.9975E q 03 113.1 " 5.5 Sample mass s 0.1280 g
J-value s 0.0014789 " 0.000037193
Integrated date s 129.8 " 3.4 Ma
Ž .
Plateau date s 130.9 " 3.4 Ma 550–11608C
DB49B plagioclase, Gabbro, Zhujiapu, Dabie Mt.
550 0.048 53.756 0.3134E q 00 0.2223E q 02 0.1116E q 00 0.1692E q 03 0.5399E q 03 200.2 " 6.3 620 0.111 27.021 0.5465E y 01 0.1504E q 00 0.3510E y 01 0.5975E q 02 0.1093E q 04 112.7 " 6.5 700 0.192 9.035 0.2796E y 01 0.4928E q 02 0.1264E y 01 0.5038E q 02 0.1802E q 04 122.0 " 7.2 800 0.283 0.000 0.9945E y 02 0.5162E q 02 0.2087E y 01 0.4825E q 02 0.4852E q 04 128.5 " 6.2 900 0.363 0.000 0.6844E y 02 0.6504E q 02 0.1380E y 01 0.4338E q 02 0.6339E q 04 116.9 " 8.1 1000 0.451 0.000 0.9125E y 03 0.7105E q 01 0.7081E y 02 0.4330E q 02 0.4745E q 05 112.4 " 5.8 1060 0.532 7.442 0.1563E y 01 0.1783E q 02 0.1993E y 01 0.4405E q 02 0.2818E q 04 106.8 " 4.8 1120 0.665 4.627 0.1798E y 01 0.4131E q 02 0.2164E y 01 0.4761E q 02 0.2647E q 04 120.3 " 4.4 1180 0.932 15.349 0.3225E y 01 0.1716E q 02 0.1998E y 01 0.5368E q 02 0.1665E q 04 118.8 " 3.8 1240 1.000 6.298 0.2497E y 01 0.5793E q 02 0.3082E y 01 0.4790E q 02 0.1918E q 04 120.3 " 5.6 Sample mass s 0.0552 g
J-value s 0.0014789 " 0.000037193
Integrated date s 122.1 " 3.3 Ma Plateau date s 118.0 " 3.2 Ma
40 39
Ž . Ž .
Fig. 9. Ar– Ar plateau ages of a an anothosite WR, b a
Ž .
hornblende and c a plagioclase of gabbro from Zhujiapu.
Ž
with large uncertainties. If such rock pyroxenite
.
BJ95-03 is excluded, then the range of TDM is reduced to 3.5–1.8 Ga, which, coincidentally, is
identical to that of Cretaceous granitic intrusions as well as older granitic gneisses from the Dabie terrane
ŽChen and Jahn, 1998 ..
From an extensive compilation of Sm–Nd iso-topic compositions and TDM values for intrusive granitoids, sedimentary and metamorphic rocks, Chen
Ž .
and Jahn 1998 demonstrated that the Yangtze– Cathaysia craton is essentially made up of Protero-zoic rocks, at least for the middle to upper crustal levels. The occurrence of an Archean granitic gneiss
Ž
in the Kongling Group in Hubei Province Zheng et
.
al., 1991; Ames et al., 1996 is very unusual, and its tectonic significance is at best ambiguous. However,
Ž .
the presence of Archean model ages s 2.5 Ga in the Dabie terrane is something unusual and may be indicative of exposure of an ancient lower crust of the Yangtze craton whose age could be Archean, or of slices of Archean crust from the Sino-Korean craton that were mixed up in the exhumation of the Dabie UHP rocks. At any rate, the Dabie orogen appears to contain some Archean crustal protolith components.
Ž . Ž .
In an ´Nd T vs. ISr diagram Fig. 10a , the data of mafic and ultramafic intrusions from both NDC and North Huaiyang Belt lie in the enriched exten-sion of the ‘mantle array’ defined by mantle-derived
Ž
rocks. The Cretaceous granitoids of Dabieshan Zhou
.
et al., 1995b,c; Xie et al., 1996 occupy a field that overlaps the mafic–ultramafic rocks. They all are
Ž .
characterized by ´Nd T of y15 to y20 and ISr of 0.707 to 0.710. The data are distinguished from the UHP mafic–ultramafic rocks, such as those from
Ž .
Rizhao, Bixiling and Maowu Fig. 10a . The geo-chemical argument presented earlier required a sig-nificant role of crustal contamination either in the mantle sources or during magma ascent and differen-tiation.
From the Nd–Sr isotopic consideration, the upper crust is not likely a good candidate as the major
Ž .
contaminant Fig. 10a . In the apparent absence of
Ž
very old upper crust in the Yangtze craton Chen and
.
Jahn, 1998 , the best candidate for crustal contami-nant is the subducted lower crust. We do not know exactly its isotopic characteristics, but a good guess
Ž .
from our compilation work Chen and Jahn, 1998 would place it as marked ‘Yangtze lower crust’ in Fig. 10a. The single data point of Kongling gneiss is
Ž .
Ž . Ž . 87 86
Fig. 10. a ´NdT vs. initial Srr Sr plot for a variety of mafic–ultramafic rocks from the Dabie orogen. Those from Rizhao, Rongcheng, Maowu and Bixiling represent UHP assemblages formed at f 220 Ma during the Triassic collision. Those from the NDC are
Ž . Ž .
characterized by the highly negative ´NdT values y15 to y20 , identical to that of the Cretaceous granitic intrusions. Possible isotopic
Ž . Ž
fields for different crustal segments are shown for comparison. Data sources: Rizhao, Rongcheng Jahn, 1998 , Bixiling Chavagnac and
. Ž . Ž .
Jahn, 1996 , Maowu Jahn et al., 1999b , Cretaceous granites Zhou et al., 1995b,c; Xie et al., 1996; Chen and Jahn, 1998 , N. Huaiyang
Ž . Ž . Ž . Ž .
gabbros Zhou et al., 1995c , Kongling gneiss Ames et al., 1996 . Other fields literature data . b Mixing calculations for source and melt contaminations. The mixing parameters used are:
UM Basalt UCC MCC-LCC 87 86 Srr Sr 0.703 0.704 0.720 0.710 wSr ppmx 20 150 350 300 ´Nd q8 q8 y10 y30 wSm ppmx 0.42 3.5 5.2 4.8 wNd ppmx 1.2 15 26 24 SrrNd 16.7 10 13.5 12.5 Ž Ž ..
UM denotes upper mantle peridotites, UCC, upper continental crust elemental data from Taylor and McLennan 1985 , MCC-LCC, middle
Ž Ž ..
to lower crust data of middle crust from Rudnick and Fountain 1995 .
but we do not think that it is representative of the Yangtze craton due to its singularity. In addition,
available Pb isotope data for the Cretaceous granitic
Ž .
their intraplate affinity and unradiogenic middle-lower crustal origin.
Ž .
Highly negative ´Nd T values for mantle-derived
mafic–ultramafic rocks have also been found in the
Ž .
late Proterozoic f 670 Ma Dovyren layered
intru-Ž
sion in northern Baikal region, Russia Amelin et al.,
.
1996 . Gabbro, troctolite, gabbronorite, dunite, pla-gioclase peridotite, and diabasic sill are the
lithologi-Ž .
cal varieties, and they all have ´Nd T values of about y14 to y15 and ISr from 0.710 to 0.714.
Ž .
Using geochemical and isotopic Nd–Sr–Pb
argu-Ž .
ments, Amelin et al. 1996 concluded that the rocks were most likely produced by partial melting of depleted lherzolite which was contaminated by
sub-Ž .
ducted sediments or continental crust prior to the melting event.
For the present case, we believe that crustal con-tamination in the mantle source is also the most probable interpretation for the isotopic and geochem-ical features. A simple modeling for source
contami-Ž .
nation Fig. 10b indicates that mixing the upper mantle peridotite with about 10% of subducted conti-nental mass with middle to lower crustal character-istics would suffice to explain the present Sr–Nd isotopic features of the mafic–ultramafic intrusions. Injection of 10% of crustal materials into the mantle would not change much the major element composi-tion, but it could significantly increase trace element abundances of the mantle peridotites. In so far as the higher Sr isotopic ratios of the Cretaceous granites
Ž .
are concerned Fig. 10a , it appears that their sources might have involved some upper crustal materials.
Crustal contamination during magma ascent might have occurred, but it cannot explain the observed isotopic and geochemical features from the mass balance consideration. The isotopic constraint favors the middle to lower but not the upper crustal contam-ination. The composition and lithological nature of the lower crust has long been debated. It is com-posed of rocks in the granulite facies and is
lithologi-Ž .
cally heterogeneous Rudnick and Fountain, 1995 . If it is of normal mafic or basaltic composition, it is unlikely to have developed a highly negative ´Nd
composition as required. If it is of intermediate composition and characterised by light rare earth enrichment, it must be old, likely to be of early Proterozoic or Archean age. It is such composition
Ž
that is used in the present calculation see Fig. 10b
.
for parameters . Fig. 10b shows two mixing curves of a basaltic magma subjected to an upper and a middle-lower crustal contamination. However, in or-der to change the Nd isotopic composition of a
Ž
mantle-derived liquid from an ´Nd of q8
assump-.
tion to y15 to y20, it requires an assimilation of 50–60% of mid-lower crustal rocks. Let alone the problem of heat budget, such a voluminous assimila-tion would severely modified the major element composition of the magmas. For example, the SiO2 contents would have to be significantly increased, but this is not the case for the gabbros and
pyroxen-Ž .
ites in question Table 1 . Note that the Sr–Nd concentrations of basalt used in the calculation are much lower than those of the gabbros, which, in our model interpretation, are derived from a metasoma-tised mantle. On the other hand, as constrained by SiO contents, a few percent of crustal contamina-2
tion in gabbroic magmas would not suffice to ex-plain the very large negative Nb anomalies in the
Ž .
spidergrams Fig. 4 or the high LarNb and BarNb ratios as shown in Fig. 5.
We therefore conclude that the present unusual isotopic compositions of the mafic–ultramafic intru-sions are a result of source mixing and provide strong evidence for crust–mantle interaction when a segment of continental crust was deeply subducted and some of it remained at mantle depth after ex-humation of UHP metamorphic rocks during the post-collisional extensional phase.
4.4. Tectonic implications
Ž .
Hacker et al. 1996 reviewed the existing tec-tonic models for the Dabie UHP terrane. The general
Ž .
agreement for all models includes: 1 the Yangtze and Sino-Korean cratons are the principal continental colliders, only some models speculatively include intervening micro-continents or intraoceanic arcs, and
Ž .2 a northward subduction of the Yangtze craton, because the fold-thrust belt in the southern part of the orogen verges southward, and metamorphic pres-sures increase northward from blueschist to coesite-bearing eclogite. By contrast, the disagreement abounds, particularly concerning the NDC. This unit
Ž .
has been regarded as 1 a less deeply subducted part
Ž . Ž .
Ž
a metamorphosed ophiolite melange complex Xu et
. Ž . Ž
al., 1992, 1994 , 3 a Paleozoic arc complex Zhai
. Ž .
et al., 1994; Wang et al., 1996 , 4 part of the Sino-Korean craton in the hanging wall of the
sub-Ž .
duction zone Liou et al., 1996; Zhang et al., 1996 ,
Ž .
or 5 extruded subduction assemblage, which in-volved a micro-continent of transition crust and the Yangtze basement, onto the Sino-Korean craton
fol-Ž
lowing an Alpine indentation model Hacker et al.,
. Ž .
1996 , or most recently, 6 a Cretaceous magmatic complex formed in the post-collisional extension
ŽHacker et al., 1998 ..
Geochemical and isotopic constraints to the tec-tonic evolution of the entire Dabie orogen may be
Ž .
summarized below: 1 the subducted continental crust is dominantly of Proterozoic ages, probably
Ž
with the Yangtze affinity Chen and Jahn, 1998;
. Ž .
Hacker et al., 1998; Jahn, 1998 , 2 the Nd isotopic compositions of UHP eclogites suggest that the sub-ducted crust was a mature, ancient and cold crust,
Ž
not of transition crustal nature Jahn et al., 1995;
. Ž .
Jahn, 1998 , 3 none of the mafic–ultramafic rocks in both central Dabie UHP and northern Dabie gneiss
Ž
terranes can be identified as ophiolite suites Jahn,
. Ž .
1998; this paper , 4 Archean isotope signature is
Ž
rarely found in any Dabie lithologic units Chen and
.
Jahn, 1998 , but this does not exclude a minor role of Sino-Korean craton, traditionally considered as of Archean age, in the making of the collisional orogen,
Ž .5 if the sediments of the North Huaiyang Flysch
Ž .
Belt s Foziling Group are considered as an accre-tionary wedge derived from the Sino-Korean craton
Ž .
in Devonian Mattauer et al., 1985 , then the hang-ingwall hypothesis is not at variance with the presently known age and isotopic data which yield
Ž
Sm–Nd TDM ages of 1.6 to 2.2 Ga Li et al., 1994;
.
Chen and Jahn, 1998; Jahn et al., 1999b .
The geochemical and isotopic analyses of mafic and ultramafic intrusions of the NDC suggest a strong interaction between mantle and subducted continental crustal rocks. The process is considered to have occurred in the post-collisional epoch, and between the hot asthenosphere and ‘trapped’ lower continental crust after most subducted continental slices were uplifted. The tectonic scenario can be illustrated by a model shown in Fig. 11.
Fig. 11a describes the early stage of collision at about 230 Ma between the Yangtze and Sino-Korean
cratons. Subduction of the continental lithosphere ceased when it reached 100–120 km of depth due mainly to its buoyancy. However, the denser oceanic lithosphere, of which the oceanic crust has dehy-drated and converted to eclogite, continued to subduct and eventually detached from the continental
litho-Ž .
sphere e.g., Davis and von Blanckenburg, 1995 . Once the detachment took place, the continental lithosphere, particularly the sialic crustal component, would have rebounded, uplifted or vertically
ex-Ž .
truded rapidly Fig. 11b , as hypothesized by Hacker
Ž .
et al. 1996 . By then UHP assemblages would have recorded their exhumation time at about 210–220
Ž
Ma Ames et al., 1993; Li et al., 1993; Hacker and Wang, 1995; Ames et al., 1996; Chavagnac and Jahn, 1996; Rowley et al., 1997; Hacker et al.,
.
1998 .
Field data indicate that, unlike the case of the Hercynian belt in western Europe, only limited vol-umes of syntectonic granites were formed during the collision and subsequent exhumation of UHP rocks. This is the most remarkable aspect of the Dabie orogen. It appears that the absence or limited amount of syn-collisional granites and the widespread preser-vation of UHP metamorphic assemblages are mainly due to the low water activity as well as the rapid rate
Ž .
of initial exhumation see also Liou et al., 1997 . The first post-tectonic granitic intrusion took place
Ž .
long almost 100 Ma after the initial collision event. Moreover, the available Sr–Nd–Pb isotopic
compo-Ž
sitions of the Cretaceous granites mainly of alkaline
.
variety indicate their intraplate affinity and
unradio-Ž
genic lower crustal origin Zhang et al., 1995; Zhou
.
et al., 1995b,c; Xie et al., 1996 .
Fig. 11c illustrates the scenario when massive granitic magmas and volumetrically much smaller but numerous basic and ultrabasic stocks, plutons or dikes were emplaced in an extensional phase at about 130–120 Ma. Heat source is vital for the apparent ‘intra-plate’ magmatisms. This can be hy-pothesized with underplating of a small plume of shallow origin, which provided sufficient heat to trigger melting of the isostatically readjusted lower crust for granitic magmas and of metasomatized
Ž
mantle melange of subducted lower crust and
man-.
tle peridotite for basic and ultrabasic magmas. Al-ternatively, the magmatic event was simply initiated by ‘internal’ heat source from the doubly thickened
radioactive crust by collision, and then aided by the asthenospheric upwelling when the thickened crust
underwent extension. We are not able to distinguish the two hypotheses of heat sources, but simultaneous
