高分异I型 Highly fractionated I-type granites in NE China (II)- isotopic geochemistry
Highly fractionated I-type granites in NE China (II):isotopic geochemistry and implications for crustal
growth in the Phanerozoic
Fu-yuan Wu a,b ,Bor-ming Jahn b,*,Simon A.Wilde c ,Ching-Hua Lo d ,
Tzen-Fu Yui e ,Qiang Lin a ,Wen-chun Ge a ,De-you Sun a
a
Department of Geology,Jilin University,79Jianshejie,130061Changchun,China
b
Ge ′osciences Rennes (CNRS),Universite ′de Rennes I,Avenue du Ge ′neral Leclerc,35042Rennes cedex,France c
Department of Applied Geology,Curtin University of Technology,Perth,Western Australia 6845,Australia d
Department of Geology,National Taiwan University,245Choushan Road,Taipei,10770,Taiwan,ROC e
Institute of Earth Sciences,Academia Sinica,P .B.Box 1-55,Nankang,Taipei 115,Taiwan,ROC
Received 13December 2001;accepted 22January 2003
Abstract
NE China is the easternmost part of the Central Asian Orogenic Belt (CAOB).The area is distinguished by widespread occurrence of Phanerozoic granitic rocks.In the companion paper (Part I),we established the Jurassic ages (184–137Ma)for three granitic plutons:Xinhuatun,Lamashan and Yiershi.We also used geochemical data to argue that these rocks are highly fractionated I-type granites.In this paper,we present Sr–Nd–O isotope data of the three plutons and 32additional samples to delineate the nature of their source,to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the Phanerozoic.
Despite their difference in emplacement age,Sr–Nd isotopic analyses reveal that these Jurassic granites have common isotopic characteristics.They all have low initial 87Sr/86Sr ratios (0.7045F 0.0015),positive e Nd (T )values (+1.3to +2.8),and young Sm–Nd model ages (720–840Ma).These characteristics are indicative of juvenile nature for these granites.Other Late Paleozoic to Mesozoic granites in this region also show the same features.Sr–Nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two-stage processes:(1)formation of parental magmas by melting of a relatively juvenile crust,which is probably a mixed lithology formed by pre-existing lower crust intruded or underplated by mantle-derived basaltic magma,and (2)extensive magmatic differentiation of the parental magmas in a slow cooling environment.
The widespread distribution of juvenile granitoids in NE China indicates a massive transfer of mantle material to the crust in a post-orogenic tectonic setting.Several recent studies have documented that juvenile granitoids of Paleozoic to Mesozoic ages are ubiquitous in the Central Asian Orogenic Belt,hence suggesting a significant growth of the continental crust in the Phanerozoic.D 2003Elsevier Science B.V .All rights reserved.
Keywords:Granite;Nd–Sr isotopes;NE China;Central Asian Orogenic Belt (CAOB);Post-orogenic;Continental growth
0024-4937/03/$-see front matter D 2003Elsevier Science B.V .All rights reserved.doi:10.1016/S0024-4937(03)00015-X
*Corresponding author.Present address:Dept.of Geosciences,National Taiwan University,245Choushan Road,Taipei,10617,Taiwan.Tel.:+886-2-2363-0231(ext.2378);fax:+886-2-2363-6095.
E-mail address:jahn@https://www.sodocs.net/doc/a47468353.html,.tw (B.Jahn)https://www.sodocs.net/doc/a47468353.html,/locate/lithos
Lithos 67(2003)191–
204
1.Introduction
Granitic magmatism is a key to the development of the continental crust and underlying lithospheric man-tle.The Central Asian Orogenic Belt(CAOB),other-wise known as the Altaid Tectonic Collage(Sengo¨r et al.,1993),is now proven to be the most important site of juvenile crustal formation in the Phanerozoic(Sen-go¨r et al.,1993;Sengo¨r and Natal’in,1996a,b;Kova-lenko et al.,1996;Han et al.,1997;Wu et al.,1998, 2000,2002;Chen et al.,2000;Heinhorst et al.,2000; Hu et al.,2000;Jahn et al.,2000a,b,c,2001;Jahn, 2002).Along with other juvenile terranes along the Pacific Coast,namely,the Canadian Cordilleran bath-oliths(Samson et al.,1989,1991;Silver and Chap-pell,1988),the Peninsular Range and the Sierra Nevada batholiths in the United States and Mexico (DePaolo,1981,1988;DePaolo et al.,1991),the Andean batholiths of South America(Kay and Rapela,1990)and the Antarctic Peninsula batholith (Pankhurst et al.,1988),the issue of continental growth in the Phanerozoic has become an important topic of research in the past decade.
Northeastern China(NE China),together with the Altai Mountains in the west and the northern part of Inner Mongolia in the middle,constitute a gigantic southern belt of the CAOB.Recent studies indicate that these granites have very juvenile,mantle-domi-nated Sr–Nd isotopic characteristics,hence suggest-ing a massive addition of new crust in this part of the world(Wu et al.,1998,2000,2001,2002;Chen et al.,2000;Chen and Jahn,2000a,b;Zhao et al., 2000;Jahn et al.,2000a,b,c;Jahn,2002).Precisely because of this special significance,we have deci-ded to conduct a more systematic isotopic and petrogenetic study on the granitic intrusions in NE China(Jahn et al.,2001;Wu et al.,2000,2001, 2002).
In the companion paper(Wu et al.,2003),we determined the emplacement ages and cooling rates of three highly fractionated I-type granitic plutons using zircon U–Pb,Rb–Sr and Ar–Ar geochron-ometers.We also used major and trace element data to constrain their petrogenesis.In this paper,we present the results of Sr–Nd–O isotopic tracer study on these three plutons.These data are used to delineate the nature of their source,and to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids.Finally,we discuss the problem of juvenile crustal growth during the Phanerozoic in conjunction with the new Sr–Nd isotopic data obtained for additional13Mesozoic granitoids from the Songliao Block and19Silurian to Cretaceous diotitic to granitic rocks from the Xing’an Block.The geological background informa-tion can be found in the companion paper(Wu et al., 2003).
2.Analytical techniques and data presentation 2.1.Nd isotope analyses and model age calculation
Nd isotopic data were obtained using the method described by Jahn et al.(1996).Mass analyses were performed using a7-collector Finnigan MAT-262mass spectrometer in dynamic mode.143Nd/144Nd ratio were normalized against the value of146Nd/144Nd=0.7219. During the period of data acquisition,our internal Ames Nd standard gave143Nd/144Nd=0.511966F7 (2r,n=20),which is equivalent to the La Jolla Nd standard of0.511860.
The notations of e Nd and f Sm/Nd are defined as:
e Nd?
?
e143Nd=144NdTs=e143Nd=144NdTCHURà1
??10000
f Sm=Nd?
?
e147Sm=144NdTs=e147Sm=144NdTCHUR
?
à1
where s=sample,and(143Nd/144Nd)CHUR=0.512638, and(147Sm/144Nd)CHUR=0.1967.Single-stage depleted mantle Sm–Nd model ages(T DM)were calculated assuming a linear evolution of isotopic composition from e Nd(T)=0at4.56Ga to+10at the present time.The equation for the single-stage model age is:
T DM?1=k ln f1t?e143Nd=144NdTsà0:51315
=?e147Sm=144NdT
s
à0:2137 g;
where k=decay constants of147Sm=0.00654 Gaà1.T DM ages of the same pluton may show a con-siderable range as a result of REE fractionation.This is best exemplified by the granites from the Xinhuatun
F.Wu et al./Lithos67(2003)191–204 192
pluton (Fig.1).In order to circumvent such a problem,we also calculate two-stage Nd model ages (T DM2)using the same assumption as Keto and Jacobsen (1987):
T DM2?T DM1àeT DM1àt Teef cc àf s T=ef cc àf DM TT;where f cc ,f s ,f DM =f Sm/Nd values of the continental crust,sample and depleted mantle,respectively.In our calculation,f cc =à0.4,f DM =0.0859,and t =intru-sive age of granite.
In fact,the choice of single-or two-stage model (DePaolo et al.,1991)is difficult,as each model has its own pitfalls and uncertainties.For the single-stage model,the main uncertainties may result from:(1)Sm/Nd fractionation between granitic melts and their sources during partial melting,(2)Sm/Nd fractiona-tion during magma differentiation,and (3)mixing of melts or sources in the petrogenetic processes (for more detailed discussion,see Arndt and Goldstein,1987;Jahn et al.,1990).Many silica-rich granitoids in NE China show highly fractionated REE patterns,leading to enhanced Nd/Sm ratios and reduced model ages.In such a case,single-stage model ages are evidently not meaningful.One way to ?correct?the problem is to restrict the use of single-stage model ages to rocks with a limited range of Sm/Nd fractio-nation,expressed as f Sm/Nd value,to the range of à0.2to à0.6.On the other hand,the two-stage model
assumes that the sources of all granites follow the same isotope evolution as the average continental crust,regardless of their true lithological character-istics.If this model is adopted,we observe that most granitoid data would form a linear array in e Nd (T )vs.T DM plots.The assumed uniform Sm/Nd for all kinds of protoliths can hardly be proven true.Nevertheless,in order to utilize all our Sm–Nd data,we chose to use T DM2in the following discussion.2.2.Oxygen isotope analyses
Quartz and feldspar were separated and purified by magnetic separation and hand picking.The purity of all mineral separates was checked by X-ray diffrac-tion,and is better than 95%.Oxygen isotope analyses were performed on mineral separates and whole rock powders using the BrF 5procedures (Clayton and Mayeda,1963).Isotope measurements were done on CO 2gas samples using a MAT 252mass spectrometer at the Academia Sinica in Taiwan.The results are reported as conventional per mil d 18O values relative to SMOW.The reproducibility is better than F 0.2x .The mean value for the NBS-28standard obtained during the present study was +9.6x .
3.Isotopic constraints on granite petrogenesis Rb–Sr isotopic data were given in Table 4of the companion paper (Wu et al.,2003).It is shown that most initial 87Sr/86Sr ratios for low Rb/Sr samples have a restricted range from 0.704to 0.705,which is rather low for granitic rocks formed in Phanerozoic orogenic belts,but is quite typical for the Phanerozoic granitoids of the CAOB (Kovalenko et al.,1996;Jahn et al.,2000a,b,c).
For the Sm–Nd isotope data (Table 1),all samples have positive e Nd (T )values (+2.5to +2.8for Xin-huatun,+1.3to +1.7for Lamashan,and +1.9to +2.5for Yiershi)and very young depleted-mantle model ages (T DM2)ranging from 720to 840Ma (except for enclave X-5).In contrast to the rocks of the Xinhuatun pluton,the granites of the Lamashan and Yiershi plutons have very low values of f Sm/Nd (down to à0.68).These values are much lower than that of the continental crust (à0.40;e.g.,Jahn and Condie,1995),and are likely produced by
extensive
Fig. 1.Differentiation index (DI,sum of normative minerals Q +Ab +Or)vs.single-stage model age (T DM1)diagram for the granites of the Xinhuatun pluton showing the effect of magmatic fractionation on calculated T DM .
F .Wu et al./Lithos 67(2003)191–204193
fractional crystallization involving high Sm/Nd acces-sory minerals,such as apatite,titanite and hornblende.To account for the petrogenesis of these highly fractionated I-type granites,several processes can be envisaged:(a)differentiation of mantle-derived mafic magma;(b)partial melting of a mixed source rock produced by intercalation of underplated mafic magma in lower crustal rocks,and (c)mixing of mantle-and crustally derived magmas,which,in turn,underwent fractional crystallization.Here we use Sr,Nd and oxygen isotope tracers to assess the different petrogenetic processes.3.1.Sr–Nd isotopic constraint
As just mentioned,the most striking feature for the granites of NE China is their low (87Sr/86Sr)i ,positive e Nd (T )values,and young Sm–Nd T DM2ages (mainly 720–840Ma)(Fig.2).In order to provide a more generalized picture for NE China,we further analyzed 32additional granitoid samples for their Sr–Nd iso-topic compositions.The localities of these samples can be identified in a later section (Fig.7).The new data are presented in Table 2.
In the Songliao Block,most of the granites were emplaced during Late Triassic to Middle Jurassic,and all of them have low (87Sr/86Sr)i ratios (ca.0.705)and positive e Nd (T )values (0to +4)except the Renao pluton situated in the south of Changchun in Jilin province.By contrast,the granites in the Xing’an Block show a larger range of emplacement ages from 430to 110Ma.The Silurian Tafeng pluton located in the northern part show negative e Nd (T )values (à1.6to à4.5)and slightly higher (87Sr/86Sr)i ratios of 0.705–0.708(except a high Rb/Sr granite),but the Permian to Mesozoic granites have the same Sr–Nd isotopic features as those in the Songliao Block.
In summary,the granites from different blocks of NE China display almost the same Sr–Nd isotopic
Table 1
Sm –Nd isotopic compositions of granites from three plutons in NE China
Analysis no.
Sample no.Age (Ma)Sm (ppm)Nd (ppm)147
Sm/144
Nd 143Nd/144
Nd
F 2r m e Nd (0)e Nd (T )T DM1(Ma)T DM2(Ma)f Sm/Nd Xinhuatun
12970X-1173 1.357.100.11520.51267470.7 2.5737754à0.4112971X-2173 3.6222.290.09820.51266290.5 2.6645742à0.5012972X-3173 2.4713.390.11160.51268660.9 2.8693728à0.4312973X-4173 6.4241.670.09320.51267070.6 2.9608721à0.5312974X-5173 4.6019.130.14520.5126888 1.0 2.11028785à0.2612975X-6173 2.8213.160.12960.5127008 1.2 2.7816738à0.3412976X-7173 3.1714.530.13170.5127096 1.4 2.8820728à0.33Lamashan
12977L-1154 3.8223.050.10010.5126256à0.3 1.6705808à0.4912978L-2154 3.4720.760.10120.5126168à0.4 1.4724824à0.4912979L-3154 3.6421.500.10240.5126157à0.4 1.4733828à0.4812980L-4154 1.9313.370.08710.5126077à0.6 1.6654816à0.5612981L-5154 1.6511.720.08510.5126017à0.7 1.5651823à0.5712982L-6154 1.9614.240.08310.5125917à0.9 1.3653835à0.5812983L-7154 1.7412.500.08400.5126067à0.6 1.6640813à0.5712984L-8154 1.067.770.08280.5125977à0.8 1.4645825à0.5812985L-91540.89 6.920.07820.5125837à1.1 1.3638840à0.60Yiershi
12986YE-1143 3.3119.940.10040.51264270.1 1.8684784à0.4912987YE-2143 1.9413.410.08750.51264970.2 2.2606754à0.5612988YE-3143 3.5722.410.09630.51267070.6 2.5624733à0.5112989YE-4143 3.3120.710.09670.51266060.4 2.3639750à0.5112990YE-5143 4.1623.810.10560.51267770.8 2.4668736à0.4612991YE-61430.24 2.300.06360.5126187à0.4 2.0541768à0.6812992YE-71430.25 2.060.07460.5126197à0.4 1.9583782à0.6212993YE-8143 2.3015.340.09060.51264070.0 2.0632773à0.5412974
YE-9
143
2.62
15.07
0.1050
0.512663
8
0.5
2.2
684
757
à0.47
F .Wu et al./Lithos 67(2003)191–204
194
characters except the Silurian Tafeng pluton.The results suggest a high proportion of mantle-derived material in the generation of these granitoids.In order to estimate the proportion of mantle-to-crust component,a simple mixing model was employed,and the result of mixing calculation using Sr–Nd isotopic data is shown in Fig.3.The proportion of the mantle component using Nd isotope data alone is illustrated in Fig.4.Fig.3shows that the upper crustal component (UCC)has little or no role in the generation of the granites;whereas mantle-derived
basaltic magma and the lower crust (LCC)are the two major components.In both Figs.3and 4,the mantle component represents about 60–90%.This by no means indicates that the granites were formed by mixing basaltic and lower crustal melts in such proportions.Rather,it suggests that the granitic magmas were produced by melting of a mixed lithology containing a lower crustal gneiss intruded or underplated by a basaltic magma in such a proportion (60–90%)for the latter.
Fig.4shows that the granites from the Songliao Block contain about 80–90%of juvenile crust,and those from the Xing’an Block are about 60–90%.Apparently,the granites from the Songliao Block have a higher proportion of the mantle component that those from the Xing’an Block.This point is in fact consistent with the oxygen isotopic data to be pre-sented below.In any case,it could be concluded that juvenile crust is the major protolith for the granites in NE China.
3.2.Oxygen isotopic constraint
d 18O data for whole-rock,quartz and feldspar samples of th
e three plutons are given in Table 3.In all cases,whole-rock and feldspar data show a greater variation in O-isotope composition than quartz.Two whole rock and four feldspar samples even have negative d 18O values.Under equilibrium conditions,the O-isotope fractionation between quartz and feldspar should fall in the range o
f 0.5–2.0x at magmatic temperatures (Chiba et al.,1989).However,the D 18O (quartz-feldspar)values range from à1.3to +3.7x ,à0.4to +11.7x ,and +1.1to +12.7x for the Xinhuatun,Lamashan,and Yiershi plutons,respectively,indicatin
g that the O isotopes did not reac
h equilibrium in all samples.Isotopic disequilibrium among minerals is not uncommon in granites.The present isotopic charac-teristics demonstrate that the three plutons have experienced post-emplacement open-system hydro-thermal alteration (Gregory and Criss,1986;Gregory et al.,1989).Meteoric water was the most probable fluid involved in the water–rock interactions for these granitic intrusions.
In a d 18O (feldspar)vs.d 18O (quartz)diagram (Fig.5),following Gregory and Criss (1986)and Gregory et al.(1989),two diagonal lines denote the
probable
Fig.2.(a)e Nd (T )vs.(87Sr/86Sr)i diagram for the granites of the Xinhuatun (open triangles),Lamashan (solid circles)and Yiershi (open squares)plutons.(b)f Sm/Nd vs.T DM diagrams showing young Sm –Nd model ages (500–1000Ma).Rocks with very low f Sm/Nd values (<à0.6)or ?high?f Sm/Nd values (>à0.2)may result in unrealistic model ages.More meaningful model ages are found with f Sm/Nd values in the range of à0.4F 0.2(in the white zone).
F .Wu et al./Lithos 67(2003)191–204195
equilibrium isotopic fractionation between quartz and feldspar at magmatic temperatures.Most data points of the Lamashan and Xinhuatun plutons fall below or above the equilibrium range.This isotopic disequili-brium is mainly due to the fact that feldspar exchanges oxygen isotopes with the fluid more readily than quartz during water–rock interaction.The data points for the Lamashan granites form a steep array,most of them below the equilibrium range,whereas those for Xinhuatun spread in an S-shape outside of the equi-librium range.The distribution and shape of such data arrays are a function of several parameters:O-isotope composition of the fluid,water–rock ratio,temper-ature,difference of reaction rate of minerals and time duration of water–rock interactions (Gregory et al.,1989).The different arrays between the Lamashan and Xinhuatun plutons,however,indicate that the O-iso-tope system in the latter is closer to the equilibrium conditions.The intersections of the disequilibrium arrays with the equilibrium fractionation lines yield d 18O values (quartz)from +6.3to +6.8x for Xinhuatun and +8.2to +8.7x for Lamashan,respectively (Fig.5).These values probably are close to the primary magmatic d 18O of quartz for these two plutons.
Similarly,in a detailed study of stable isotopes of anorogenic granitoids from Transbaikalia,Wickham et al.(1996)found that most samples older than Early Permian show ?normal?equilibrium fractio-nations with D Q àFd between 0.5and 2.5.However,disequilibrium fractionation is found in samples of Permo-Triassic and Mesozoic plutons,which define fields with steep slopes that cross-cut the equili-brium fractionation lines at high angles,similar to the Lamashan granites (Fig.5).These systematics are typical of plutonic rocks subjected to meteoric-hydrothermal alteration (e.g.,Criss and Taylor,1986).Unlike the Lamashan and Xinhuatun granites,the isotopic compositions of the Yiershi samples (except YE-1)fall close to the equilibrium lines at magmatic temperatures (Fig.5).Their d 18O values might therefore be primary.A single feldspar
(sam-
Fig.3.e Nd vs 87Sr/86Sr isotopic ratio plot showing mixing proportions between two end-members:(1)depleted mantle or juvenile components (DM =upper mantle peridotite;or B =basalt)and (2)crustal components (LCC =lower continental crust;or UCC =upper continental crust,both of them are represented by the Mashan Group gneisses in the Jiamusi Block,see detail explanation in Wu et al.,2000).The parameters used are:
DM
Basalt UCC LCC 87
Sr/86Sr 0.7030.7040.7400.708[Sr]ppm 20200250230e Nd
+8+8à12à15[Nd]ppm
1.2
15
30
20
F .Wu et al./Lithos 67(2003)191–204197
ple YE-1,Fig.5),however,has negative O-isotope composition and is clearly not in isotopic equili-brium with the coexisting quartz,a situation similar to some Lamashan samples.Nevertheless,the present dataset demonstrates that meteoric water–rock interaction for the Yiershi granites was rather restricted in space during hydrothermal alteration.Note that the d 18O values of quartz and feldspar (except YE-1)range from +7.3to +10.1x and from +6.0to +9.4x ,respectively.Although these ranges are normal for granitic rocks,they are too large to be accounted for by simple magmatic differentiation considering that the modal composi-tions of quartz and feldspar (in Table 1;Wu et al.,
2003)in these samples are similar (Sheppard,1986).Assimilation of country rocks during magma emplacement might have played an important role and the O-isotopes would not have been completely homogenized during assimilation and subsequent magmatic processes.
Therefore,the O isotopic data suggest that these granites from NE China did not come directly from the mantle but from a juvenile crust or a mantle-derived magma that had been contaminated by a preexisting crustal component.It is also shown that the Xinhuatun granites from the Songliao Block have lower d 18O values than the Lamashan and Yiershi granites in the Xing’an Block,which might indicate that the Xinhuatun contains more juvenile compo-nents than the latter.This is generally consistent
with
Fig.4.Estimated proportions of the mantle or juvenile component for the granites from NE China.The equation is x m =(Nd c /Nd m )/((Nd c /Nd m )+(e m àe s )/(e s àe c ))(DePaolo et al.,1991),where x m =%mantle component (represent by basalt).Nd c ,Nd m =Nd concen-trations in the crust and mantle components,respectively.e m ,e s and e c =Nd isotopic compositions of the mantle or juvenile crust,samples measured,and crustal component,respectively.Parameters used:e m =+8,e c =à15.Nd c =25ppm,Nd m =15ppm.
Table 3
Oxygen isotope data (in x ,relative to SMOW)Sample no.Whole-rock Quartz Feldspar D (Q àFd)Xinhuatun X-17.6 6.67.9à1.3X-2 5.4 6.5 4.2 2.3X-3 2.0 5.8 2.1 3.7X-4 3.7 6.0 2.7 3.3X-50.7 4.9 1.9 3.0X-67.87.68.5à0.9X-7 6.9
6.8
8.5
à1.7
Lamashan L-1à0.47.3à5.412.7L-2 1.88.1à1.910.0L-3 5.07.9 2.7 5.2L-4 6.48.7 6.1 2.6L-5 6.98.67.5 1.1L-6 4.08.3 2.4 5.9L-7 6.88.67.2 1.4L-8 1.68.2à0.78.9L-9 3.08.7 1.27.5
Yiershi YE-1à1.6 6.7à5.011.7YE-28.18.08.4à0.4YE-38.48.97.6 1.3YE-48.58.97.7 1.2YE-58.68.98.60.3YE-69.29.49.30.1YE-79.710.19.40.7YE-87.68.0 6.5 1.5YE-9
7.07.3 6.0 1.3
F .Wu et al./Lithos 67(2003)191–204
198
the highest e Nd(T)values(+2.1to+2.9)among the three plutons.
4.Tectonic implications
Geochemical discrimination of tectonic environ-ments for granite generation is often ambiguous and sometimes extremely controversial(Pearce et al., 1984;Maniar and Piccoli,1989;Pearce,1996).In the Nb vs.Y and Rb vs.(Y+Nb)diagrams(Fig.6), the three studied highly fractionated I-type granites of NE China fall in the volcanic-arc field of Pearce et al. (1984).According to the criteria of Sylvester(1989), however,these rocks belong to post-collisional alka-line granites.This contradiction suggests that the geochemical identification of tectonic setting is not straightforward.
For the Late Paleozoic to Mesozoic granites formed in this region,four possible tectonic settings can be hypothesized:(1)a west-dipping subduction zone of the Paleo-Pacific ocean;(2)a SE-dipping subduction zone of the Mongolia–Okhotsk ocean;(3) a post-orogenic extensional collapse of the Central Asian orogenic belt;and(4)an anorogenic setting. Usually,granites formed in subduction zones show a roughly linear distribution,which is not the case in NE China.Thus,the first two subduction
mechanisms Fig.5.Feldspar d18O versus quartz d18O diagram.Two lines with
constant D qz-fd values represent possible isotopic fractionations
between quartz and feldspar at magmatic temperatures.Open
squares:Xinhuatun pluton,open circles:Lamashan pluton,and
solid circles,Yiershi pluton.Note that samples YE-1is completely
outside of the Yiershi range.The data for the rocks from
Transbaikalia are from Wickham et al.(1996)
.
Fig.6.Tectonic discrimination diagram of Pearce et al.(1984).
Fields for Syn-COLG(Syncollisional),V AG(V olcanic arc),WPG
(Within-plate)and ORG(Ocean-ridge)granites are indicated.
F.Wu et al./Lithos67(2003)191–204199
are not favored.Although these granites could be formed in an anorogenic setting associated with man-tle plume activity as suggested by Dobretsov and Vernikovsky (2001),this hypothesis is no longer valid due to the large range of emplacement ages and the absence of intense mafic magmatism.
Combined with the result of a study on A-type granites in this region (Wu et al.,2002),we consider that the areal distribution of granites may be related to post-orogenic extensional collapse of the Xingmeng orogenic belt.In order to account for the huge volumes of granites,we suggest that granitic forma-tion was related to massive underplating of mafic magma in an extensional tectonic setting.
It has been shown geochemically that these gran-ites are in marked contrast to those of the Lachlan fold belt in Australia (Gray,1984;Chappell and White,1992)and the Caledonian and Hercynian belts in Europe (Stephens,1988;Pin and Duthou,1990;Siebel et al.,1995),but are similar to Mesozoic and younger granites of the eastern Pacific Coast,such as the Canadian Cordilleran batholiths (Samson et al.,1991;Silver and Chappell,1988),the Peninsular Range and the Sierra Nevada batholiths (DePaolo,1981,1988;DePaolo et al.,1991),the Andean bath-oliths of South America (Kay and Rapela,1990)and the Antarctic Peninsula batholith (Pankhurst et al.,1988).Our study also suggests that the granites of NE China are difficult to be related to subduction pro-cesses for the reasons given above,but more likely to post-orogenic magmatism.Magmatic compositions have been shown to be related to tectonic setting (Pearce et al.,1984;Maniar and Piccoli,1989;Rogers and Greenberg,1990),but it depends more
impor-
Fig.7.Distribution of Nd model ages (T DM2)(in Ma)in NE China.Three triangles represent the Xinhuatun,Lamashan and Yiershi plutons.The circled numbers correspond to the sample order shown in Table 2,and the number in parentheses represents the number of samples giving the average model ages.Data source:this paper;Wu et al.(2000,2001,2002).
F .Wu et al./Lithos 67(2003)191–204
200
tantly on its source rock nature and melting condi-tions.For example,S-type granites in the Hercynian belt of Western Europe were thought to be produced in a collisional or subduction event(Siebel et al., 1995),but other studies suggest that they were closely related to post-orogenic underplating(Downes et al., 1990;Williamson et al.,1992;Costa and Rey,1995).
A study on?subduction style?magmatism in NE Washington State,USA,also suggested that the sub-duction signature was inherited from the source rocks (Morris et al.,2000).Thus,attention must be paid when using the geochemical data to identify tectonic environments(e.g.,Forster et al.,1997).
5.Crustal growth
Granite is the major component of the continental crust on Earth,hence the growth of the continent hinges much on the mode of generation of granitoids rocks.Fig.7summarizes the spatial distribution of Sm–Nd model ages(T DM2)for the granitoids of NE China.The samples from the Jiamusi Block,a Proter-ozoic microcontinent,have much older model ages of about1600Ma(Wu et al.,2000).However,in the Songliao and Xing’an Blocks,most samples show model ages younger than1000Ma,clearly indicating a juvenile nature of the crust in this area.
Traditionally,continental crustal growth is known to take place along continental margins by accretion of island arcs,back arc basins,and by intrusion of continental arc batholiths that comprise,in part, mantle-derived magmas(Reymer and Schubert, 1986;Rudnick,1995).Less easy to assess is the extent of addition through underplating and intrusion of mantle-derived magmas in continental interiors during extension(V oshage et al.,1990;Hilderth et al.,1991;Coffin and Eldholm,1994;Stein and Hofmann,1994;Albarede,1998;Condie,1999;Frost et al.,2001).We argue that the formation of Phaner-ozoic granites in NE China was related to basaltic underplating in association with post-orogenic pro-cesses.Several petrogenetic consequences may be envisaged:(a)supply of thermal energy for promot-ing crustal melting,perhaps via repeated sill injec-tion;(b)remelting the earlier underplated mafic crust via further underplating or sill injection;(c)extensive crystal fractionation leading to formation of silicic magmas;(d)assimilation of pre-existing crust,and then fractionation to more silicic magmas.Because the amounts of granitic magma generated from underplated mafic rocks by partial melting is smaller than the source itself,the amount of underplated basaltic magma should be much greater than the volume of granite.Consequently,we believe that basaltic underplating is as important a mechanism as the subduction zone processes in the growth of the continental crust.
6.Conclusions
The present study leads to the following conclu-sions:
(1)The highly fractionated I-type granites of the
Xinhuatun,Lamashan and Yiershi pluton are characterized by relatively juvenile isotopic com-positions:positive e Nd(T)values(+2to+3)and low(87Sr/86Sr)i(0.7045F0.0010).This indicates
a substantial contribution of mantle material in the
generation of these granites.
(2)Similar isotopic characteristics are also observed
in other Paleozoic to Mesozoic granitic plutons in this area and in other parts of Central Asia.Thus, they provide strong evidence for a significant production of juvenile crust,and hence growth of the continental crust,in the Phanerozoic.
(3)Sr–Nd and oxygen isotope data suggest that the
granites were most probably derived by melting of mixed source rocks in the lower crust produced by underplating of mantle-derived magma.Tectoni-cally,these highly fractionated I-type granites could be the result of post-orogenic magmatism.
We consider that the continental growth is achieved by both lateral(arc accretion)and vertical (underplating)processes. Acknowledgements
Fuyuan Wu is most grateful to the laboratory staff in Rennes,particularly to Odile Henin,Joe′l Mace′for their instruction in chemical separation and mass spectrometry.R.Capdevila,Jon Patchett and H.-J. Forster read an earlier draft and provided many useful
F.Wu et al./Lithos67(2003)191–204201
suggestions.A.Cocherie and O.T.Ramo reviewed the manuscript and helped improve the manuscript. This work was supported by the National Natural Science Foundation of China(NSFC grant4940008to Q.Lin and49872031to F.Y.Wu),the State Education Commission of China(to F.Y.Wu for his stay in France),and French research programmes of?Dyna-mique des Transferts Terrestres?(INSU-DTT97)and ?Cycles Ge′ochimiques?(INSU-IT99)granted to B.-M.Jahn.This is INSU Contribution No.337.This paper is also a contribution to IGCP-420:Crustal Growth in the Phanerozoic:Evidence from East-Central Asia.
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