Dec 15, 2008 - 1 Distribution diagram of the Cenozoic basalts of Tianheyong and its adjacent region, Inner Mongolia (after the 1:5000000 Igneous Geological ...
EARTH SCIENCE FRONTIERS Volume 16, Issue 2,March 2009 Online English edition of the Chinese language journal Cite this article as: Earth Science Frontiers, 2009, 16(2): 090–106.
RESEARCH PAPER
Petrogenesis and Geological Implications of the Tianheyong Cenozoic Basalts, Inner Mongolia China YANG Zongfeng , LUO Zhaohua, ZHANG Huafeng, ZHANG Yongmei, HUANG Fan, SUN Chenguang, DAI Jingen State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China
Abstract: The Tianheyong basalts are basanites, which at least can be subdivided into three types of mineral assemblages. In general, the Tianheyong basalts are characteristics of low silica (w(SiO2)=43.97%–45.68%), enriched alkali (w(K2O+Na2O) =5.91%–7.65%), enriched potassium (w(K2O)=2.04%–2.89%), high contents of titanium (w(TiO2)=2.18%–2.37%), high magnesium value (Mg#=68–76), and high contents of REE (ȈREE=(246.62–329.82)×10-6). The chondrite normalized patterns of REE show right inclined smooth lines, extremely enriched in LREE, significantly fractionated from LREE and HREE, without obvious anomaly of Eu (įEu=0.90–1.02) and Ce (įCe=0.96–1.00), and the high enrichment of incompatible elements, especially, the peak value in high field strength elements (HFSE) of Nb, Ta, and Th indicate the OIB-like distribution patterns of multi-elements. The Tianheyong basalts are also high in compatible elements: Co ((39.1–48.9)×10-6), Ni ((130–257)×10-6), and Cr ((138–320)×10-6). All of the above characteristics as well as the low degree of crystallization, the abundance in peridotite inclusions, and minor amount of xenocrysts, and the variation trend of elements indicate that the basalts were solidified from primitive basaltic magma. Trace element ratio of Ba/Rb (12–35) and variation of alkali metals might indicate the metasomatism of fluids in a source region of enriched lherzolites. Petrogenesis simulation indicates that the Tianheyong primitive basaltic magma might be the product of polybaric melting column with non-modal partial melting processes of enriched lherzolite, which straddle the boundary between garnet- and spinel-facies lherzolites. Magma might originate from low partial melting of source rocks(̚ 5%, Ne > 5%, and it can be grouped into potassic basanite-basanite (Table 1). CaO contents show a little increase with increasing Al2O3 contents (data from this paper) or are at the same level with that of Al2O3 contents (data in reference[6]); however, the plots show dispersal in other region without significant correlation, which probably indicate that the magmatic processes of differentiation, assimilation, and magma mixing might not be responsible for the compositional variation of these basalts. For the Tianheyong basalts, the correlation of CaO vs Al2O3 might be from the fractionation of minerals with low contents or high contents in CaO and Al2O3, which might show that magnesium-rich olivine- or plagioclase and clinopyroxene-
fractional crystallization must have taken place. But, according to petrographic study, the simultaneous fractional crystallization of plagioclase and clinopyroxene is unlikely realistic because the main mineral near the liquidus of the Tianheyong basaltic melt is olivine. t Fe2 O 3 show little co-variation with TiO2, the positive correlation occurred in Datong, Yangyuan, and Jining basalts, which might be related to the fractional crystallization of Fe-Ti oxides. The compositional variation of the Tianheyong basalts can be distinguished from other Mesozoic and Cenozoic basalts because of its high Mg# content. There is no correlation or t little co-variation of Fe2 O 3 and Mg#, and little negative correlation of CaO and Mg# does not show the trend of fractional crystallization of olivine and clinopyroxene. The significant negative correlation of K2O and Mg# seems that fractionated olivine leads to the decrease of MgO contents and increase of K2O, but it does not correspond with the high magnesium (Mg# > 64). No co-variation of K2O vs SiO2, and silica contents in olivine are less than those of whole rock in the Tianheyong basalts. So this deduction might not be realistic and the co-variation of K2O and Mg# is most probably resulted from partial melting processes but not from fractional crystallization. K2O and Na2O content changes in a wide range,
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
especially, significant changes in Na2O contents. Alkali co-variation with Mg# might show different degree of partial melting because potassium and sodium are apt to enter into the melts during partial melting processes or influenced by the metasomatic processes of alkaline fluids. The co-variation of SiO2 and Mg# also show little differentiation and evolution trend among these basalts. Some evidence of the contributions of partial melting degree to the magmatic evolution of the Cenozoic basalts is clearly displayed by the co-variation of SiO2 and Mg# in the Datong, Yangyuan, and Jining basalts. The non-correlation or negative correlation between P2O5 and Mg# indicate that the phosphor-content are also not correlated with the process of crystallization differentiation. 4.2
Trace elements
The Tianheyong basalts are high in REE contents (Table 2), í while ȈREE ranges between (246.62–329.82)×10 6, with an í average of 281.34×10 6, similar to or higher than that of the Cenozoic alkaline basalts. Chondrite-normalized REE patterns are smooth right inclined lines, with strong enrichment of LREE. Values of Ȧ(LREE)/Ȧ(HREE) varied from 10.90 to 11.58, with an average of 11.24. (La/Yb)N=27.6–32.9, with an average of 31.7, and LaN=157–217, with an average of 184. The fractionation of HREE and LREE are obvious, and the fractionation of HREE is stronger than that of OIB, (La/Sm)N=2.8–3.2, and (Gd/Lu)N=7.64–9.34. The depleted HREE indicates that the garnet exists as the residual phase or very low degree of partial melting of the mantle source regions. All the rocks show no Eu and Ce anomaly, įEu=0.90–1.02, įCe=0.96–0.99, and the lack of Eu anomaly indicates no plagioclase fraction (Fig. 5), which coincide with the analysis of major elements and petrographical characteristics. Chondrite-normalized multi-elements patterns of the Cenozoic basalts from Tianheyong and adjacent regions, no matter alkaline or tholeiitic, are all similar to those of OIB[17–19], and well consistent with the spider diagram of Jianguo (106 Ma) basalts[20] in Liaoning Province, except its
higher concentration of incompatible elements involving a peak value in high field strength element (HFSE) of Nb, Ta, and Th. Negative anomaly of potassium might be obtained from potassium-rich minerals present as the residual phase or lack of potassium in the source regions. Most of the trace elements make up a series of parallel lines indicating relative homogeneity of material in the source region, without Sr negative anomaly, which also excludes plagioclase fractional crystallization, and negative anomaly and conspicuous intersecting of Ba and Rb might be related to the controlling factor of the fluid metasomatic processes (Fig. 5). 4.3
Spatial variation of elements
Compositional variation of magma might be occurred during the processes of magma transportation in the conduit or magma flow after eruption onto the surface: (1) The lower part of the melt column would contact with the wall-rock for a longer span of time and are easily influenced by the assimilation-fractional crystallization (AFC) process; (2) Larger density minerals (e.g. Olivine, Fe-Ti oxides) are apt to accumulate at the bottom of the melt column, smaller density minerals (e.g., Plagioclase), melt residuals, and volatiles are easily to be enriched in the upper part. Accordingly, we can use the spatial variation of chemical elements to investigate the variation of chemical composition during magmatic processes. Figure 6 displays some of the features of the chemical variation trends from south to north. Several major and trace element variation trends from south to the north shown in Fig. 6 are as follows: (1) The concentrations of FeO and MgO decrease simultaneously ; (2) Little variation of CaO contents; (3) Fe2O3,TiO2, and Al2O3 contents increase very rapidly; (4) Na2O and P2O5 decrease first and then increase rapidly; (5) Little variation of K2O contents in the south part and decrease significantly at the last four points of the north part. Correspondingly, compatible elements of Ni and Cr contents decrease consistently with the variation trend of FeO and MgO, respectively, which seemingly indicates the olivine fractional crystallization.
Fig. 5 Chondrite-normalized trace element patterns of the Tianheyong basalts Chondrite-normalized value from references [21] and [22] for left and right diagrams respectively. OIB value from references [21]. Average value of the Jianguo basalts from references [23] and [24]
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
Fig. 6 Spatial distribution of elements in Tianheyong basalts Major elements unit is %, trace elements unit is ×10-6, Ol and Ne represent the Olivine and Nepheline CIPW contents respectively. Distance from south to north indicate the distance between every two sample point from south to north, detailed information of the sample spatial relationship in Fig. 1c
However, the little variation of CaO, K2O contents, and Mg# don’t support this deduction. Little variation of įEu consistent with the little variation of CaO contents indicates no plagioclase being brought in or out. ȈREE and some incompatible element (Zr, Nb, Ta, Th, Hf, La, Yb) contents increase from south to north consistent with Fe2O3, TiO2, and Al2O3. The variation trend of Sc is somewhat special, which is a bit similar to K2O. The variation trend of La/Nb value is analogous to that of incompatible elements, values of Rb/Sr, Nb/Ta, and Zr/Hf show no obvious changes. The Ba/Rb value and (La/Yb)N changes significantly but scattered beyond linearity. So, it seems that the compositional variation trends of the Tianheyong basalts are not resulted from AFC processes, and it is likely that the variable degree of partial melting (see discussion below).
5
Discussion
As discussed above, the Tianheyong basalts are small in volume and with only one layer but have three different mineral crystallization sequences: (1) Olivine ĺ Olivine +
Clinopyroxene ĺ Clinopyroxene + Fe-Ti Oxides + Plagioclase ĺ Palagonite (Fig. 2a). (2) Olivine ĺ Olivine + Plagioclase ĺ Plagioclase + Clinopyroxene + Fe-Ti Oxides ĺ Volcanic glass (Fig. 2b). (3) Olivine ĺ Olivine + Plagioclase ĺ Plagioclase + Clinopyroxene + Fe-Ti Oxides ĺPlagioclase + Volcanic glass (Fig. 2c), which are also supported by the An-Di-Fo ternary diagram in chemical compositions (Fig. 7), and indicate that the Tianheyong basalts are products of at least three different primitive magmas and are consistent with the spatial variation of chemical compositions displayed above and can not be explained from the AFC processes. 5.1
Identification of primitive magma
Primitive magma is defined as the magma that equilibrate with the residual phases assemblage from the source region[1]. Rocks solidified from the rapidly erupted magma and without any changes in composition can represent the compositions of primitive magma. Because of little changes of the major element contents in the mantle peridotites[27], we can investigate the primitive property of the basalts according to
Table 1 Major element composition and CIPW contents of Tianheyong Basalts Sample
wB/10í6 TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Total
Or
Ab
An
Ne
Di
Ol
Mt
He
Il
Ap
06THY02
44.80
44.31
14.37
7.41
4.59
0.16
8.29
6.71
5.38
2.04
1.32
2.00
99.89
12.38
25.56
9.27
11.47
12.50
10.79
8.71
1.60
4.60
3.14
76.30
06THY04
2.36
2.37
14.43
6.90
5.01
0.16
6.10
8.11
5.00
2.62
1.38
2.86
99.76
16.06
20.24
9.53
12.81
17.77
5.27
10.15
0.16
4.68
3.33
68.46
06THY06
45.00
2.35
14.23
5.62
6.07
0.16
8.00
6.99
5.11
2.50
1.32
1.60
99.56
15.18
21.38
8.74
12.48
14.24
11.89
8.37
-
4.59
3.14
70.14
06THY08
45.68
2.33
14.13
6.77
5.13
0.16
7.38
6.98
5.31
2.34
1.32
1.65
99.68
14.18
25.92
8.00
10.92
14.58
8.69
10.06
-
4.53
3.13
71.95
06THY09
44.38
2.32
13.69
4.23
7.30
0.16
8.99
6.94
4.25
2.79
1.27
2.45
99.50
17.12
17.59
10.42
10.70
13.34
16.85
6.37
-
4.57
3.06
68.70
06THY11
44.65
2.21
13.62
3.56
7.62
0.15
9.48
6.68
3.96
2.76
1.20
2.91
99.56
17.01
18.60
11.72
8.86
11.77
19.40
5.38
-
4.37
2.89
68.92
06THY12
44.61
2.18
13.51
3.30
7.91
0.15
9.78
6.75
3.69
2.72
1.16
3.05
99.60
16.78
18.47
12.81
7.66
11.43
20.73
4.99
-
4.32
2.80
68.79
06THY13
43.97
2.19
13.31
3.56
7.80
0.15
9.45
6.73
4.57
2.89
1.21
2.90
99.52
17.82
13.97
7.59
14.29
15.23
18.45
5.39
-
4.34
2.93
68.35
06THY14
44.36
2.18
13.33
3.59
7.62
0.15
9.78
6.56
4.41
2.87
1.21
2.70
99.51
17.66
15.72
8.43
12.53
13.79
19.24
5.42
-
4.30
2.92
69.59
#
2+
2+
2+
Mg =100xMg /x(Mg +Fe ), CIPW calculated by SINCLAS software
[25]
Mg#
.
Table 2 Trace element composition in Tianheyong Basalts Samples THY02
wB/10í6 Sc
Cr
Co
Ni
Cu
Zn
Ga
Rb
Sr
Y
Zr
Nb
Mo
Sn
Cs
Ba
La
Ce
Pr
Nd
Sm
9.5
138
39.1
130
29
136
23
22
1311
22
315
94
6.5
2.7
0.5
364
65
133
15.4
71
14.3
THY04
10.6
150
40.6
142
32
149
25
33
1983
24
327
97
6.1
2.8
0.5
484
67
132
15.2
70
13.9
THY06
11.4
150
43.5
162
31
149
25
23
1398
23
332
96
6.6
2.9
0.5
373
62
129
14.4
66
13.1
THY08
12.3
234
46.6
208
30
145
25
15
1341
22
321
95
3.4
2.7
0.5
371
60
118
13.5
64
12.2
THY09
12.9
223
45.8
202
32
140
24
26
1328
21
299
91
6.8
2.5
0.4
383
54
108
12.6
58
11.4 10.9
THY11
12.8
239
47.5
235
35
140
25
26
1525
21
305
92
7.1
2.6
0.4
904
52
105
11.9
56
THY12
12.1
275
48.4
240
35
146
24
25
1480
21
299
91
6.6
2.5
0.4
498
49
100
11.4
52
10.2
THY13
12.2
274
48.0
246
34
141
25
27
1264
21
306
92
7.7
2.5
0.4
326
54
106
12.2
55
10.8
THY14
12.5
320
48.9
257
33
139
25
26
1259
21
303
92
7.1
2.5
0.4
353
51
102
11.8
54
10.8
THY14*
13.1
329
49.3
266
34
141
25
27
1282
21
307
93
7.4
2.6
0.4
360
52
103
11.8
54
10.3
Samples THY02
wB/10í6 Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
Pb
Bi
Th
U
ȈREE
įEu
˄La/Yb)N
įCe
LaN
4.1
12.1
1.7
7.9
1.22
2.7
0.28
1.4
0.18
9.3
7.1
4
0.007
10.6
3.8
329.82
0.93
32.2
0.98
209 217
THY04
4.2
11.6
1.7
7.5
1.23
2.6
0.26
1.4
0.16
9.2
6.1
4
0.016
10.1
2.7
329.38
0.98
32.9
0.96
THY06
3.7
11.4
1.6
7.0
1.08
2.4
0.26
1.3
0.16
8.6
5.9
4
0.010
9.2
3.0
313.35
0.91
32.7
1.00
201
THY08
3.6
10.2
1.5
6.8
1.02
2.3
0.25
1.2
0.16
8.9
5.9
4
0.011
9.0
1.8
294.53
0.96
32.6
0.96
193
THY09
3.3
9.9
1.3
6.4
0.96
2.2
0.22
1.2
0.16
7.9
5.4
4
0.008
7.9
2.4
269.91
0.93
30.8
0.96
174
THY11
3.5
9.7
1.4
6.0
0.91
2.2
0.24
1.1
0.16
7.6
5.2
3
0.011
7.8
2.4
259.86
1.02
31.3
0.98
167
THY12
3.1
9.3
1.4
5.5
0.92
2.1
0.22
1.2
0.15
7.5
5.3
3
0.010
7.3
2.3
246.62
0.96
27.6
0.99
157
THY13
3.2
9.5
1.3
5.9
0.89
2.1
0.24
1.1
0.16
7.8
5.4
3
0.010
7.7
2.3
261.80
0.95
34.1
0.96
174
THY14
3.0
9.3
1.3
5.6
0.89
1.9
0.22
1.1
0.13
7.6
5.2
3
0.011
7.5
2.3
252.10
0.90
31.5
0.97
163
THY14*
3.2
9.3
1.3
5.8
0.91
2.1
0.22
1.1
0.16
7.6
5.3
3
0.007
7.4
2.3
256.04
0.98
32.4
0.96
169
THY14* is parallel analysis.
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
SiO2
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
the results of melt-mineral thermodynamic equilibrium studies. For example, on the basis of the consistent partition coefficient of FeO/MgO of melts vs olivine, several discrimination standards of Mg# have been proposed for the basaltic magma, i.e., 68–72[28], 68–75[29], 64–72, and 61–75[30]. The high contents of compatible trace elements are also often used to examine the primitive magma as well, such as Sc, Cr, Co, and Ni. In addition, the mantle derived peridotite inclusions in the basalts also indicate that they are the product of primitive magma on the basis of the hydrokinetics of magma fluids. The Tianheyong basalts are abundant in abyssal peridotite inclusions[6], Mg# range from 64 to 76 (this paper and references[6]), Sc contents range from 9.5×10í6 to 12.8×10í6, Co contents range from 39.1×10í6 to 48.9×10í6, Cr contents range from 138×10í6 to 320×10í6, Ni contents range from 130×10í6 to 257×10í6, and all of these properties indicate that the Tianheyong basalts are similar to primitive basaltic magma. Very small crystal size and large amount of volcanic glass show that the basalts are quenched from rapidly erupted magma. In order to constrain the property of primitive magma and degree of partial melting, we will simulate the partial melting processes from both compatible and incompatible elements. 5.1.1
Compatible elements
Some basalts with relatively low nickel contents might not be resulted from olivine fractional crystallization supported by major element analyses above, and further explanation of quantitative calculation will be discussed below. Nickel as a compatible element supports the following equation during low degree partial melting processes: i
i
C L / CO
1/ D
i [31,32]
contents (130–257)×10í6 of the Tianheyong basalts. However, when 5% olivine is fractionated from the most primitive basaltic magma, with nickel contents of 257×10í6(THY14), it will also produce the basalts, with 130×10í6(THY02) nickel Ni
contents ( C L
F
D Ni 1
Ni
u C O =130×10í6)[35], and which will
result in the enrichment of incompatible elements, such as La
CL
F
D La 1
La
u C O =55×10í6, which is smaller than the real
contents of La (65×10í6) in the basalts. This phenomenon was very common for other basalts and incompatible elements therein, which indicates that the low nickel content of the Tianheyong basalts might result from the nickel contents of mantle peridotites, the partition coefficients of nickel, and degree of partial melting. The problem of chromium contents of the basalts can also be explained in this way. The simultaneous decrease of nickel and chromium excludes the fractional crystallization of olivine because chromium is incompatible element in olivine[36], and the fractionation of olivine alone must have caused the increase of Cr content. Figure 8 presents that the majority of the Tianheyong basalts are distributed in the lower degree partial melting line, which equilibrate with the mantle peridotites. As discussed above, some low nickel content basalts fell into the olivine fractionation curve, which might result from the compositions of the source regions and the variation in nickel partition coefficients etc. Hannuoba basalts having higher nickel contents might be related to the more enriched nickel contents of the source peridotites. The Ni vs MgO variation of the majority of the Cenozoic basalts don’t show obvious olivine fractionation apart from some Jining basalts, which fall into the olivine fractionation curve.
(1)
Ni
If D Ni =15 [33] and C O =(2000–4000)×10 í6[34] in mantle peridotites during partial melting processes, the n Ni C L =(133–266)×10 í6 , which is consistent with nickel
Fig. 8 Ni-MgO correlation diagram for partial melting and fractional crystallization models
Fig. 7 Three phase diagram of An-Di-Fo for Tianheyong basalts at 1 atm[26] The arrow in the diagram represents the crystallization path of the Tianheyong basalts. An–Anorthite; Di–Diopside; Fo–Forsterite
The line marked with 5% and 20% represents the melt from batch partial melting of mantle peridotites with 2300×10í6 nickel[31,32](suppose melt generated with wide range of MgO contents). Four curves represent Olivine fractional crystallization trend, the initial MgO contents from left to right are 8%, 12%, 16%, 20%. The Nickel partition coefficient D=(124/MgOˉ0.9)[37]. Legend same as Fig. 3
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
5.1.2
Incompatible elements
According to the model of Non-Modal partial melting:
1 /[ DO F (1 P )] [31,32]
CL / CO
(2)
where CL represents the element concentrations in the melts, CO represents the element concentrations in the primitive solid phases, DO represents the bulk partition coefficients of source rocks, F represents the degree of partial melting, and P represents the bulk partition coefficients of the melting mineral assemblage. For the two trace elements i and j: i
j
j
i
i
j
i
j
1 / C L [C O (1 P ) / C O (1 P )] [ DO (1 P )
1 / CL j
i
j
i
DO (1 P )] /[(1 P )C O ]
(3)
Proposed that: j
i
i
j
[C O (1 P ) / C O (1 P )] j
i
j
i
DO (1 P )] /[(1 P )C O ] i j i variables, C O , C O , DO
i
j
K , [ DO (1 P ) B j
The and DO are nearly constant during low degree Non-Modal partial melting. Pi and Pj might change a little bit but nearly no variation for two strong incompatible elements are present with similar partition coefficient. We deem that K and B are constant j i i j j i and K | C O / C O , B§0, that is: C L / C L | C O / C O . The trace elements with similar incompatibility support the following equation in the model of Non-Modal partial melting: i
1 / CL
j
K (1 / C L ) B
source rocks are still less than 1 and still showed incompatibility. Their co-variation with strong incompatible elements can be used to identify the process of Non-Modal partial melting and distinguish it from fractional crystallization. Sc and Th are the two particular elements and their concentrations would increase at the same time in the fractional crystallization processes (Fig. 10) because they are usually incompatible elements in olivine and plagioclase (Table 3), and Sc will decrease following the fractionation of Sc-enriched minerals (e.g., garnet and clinopyroxene), but clinopyroxene fractionation is not supported by the study of petrography, and the major element and trace element analyses. The co-variation should be ascribed to the decrease of relative content of Sc-enriched mineral (e.g., garnet and clinopyroxene) during partial melting in source region, which can result in the decrease of the total partition co-efficient of Sc (PSc) in the melt mineral assemblage. In addition, the correlation of La/Yb vs La is poor (R=0.57). It is probably related with the variation of PYb during partial melting. La is consistently incompatible for all the known minerals in the mantle peridotites, and the PLa value is hard to change clearly, whereas the PYb value was influenced significantly by the variation of the garnet relative contents
(4)
We did some linearity simulation of the stable LREE, which are similar in geochemical property and high in concentration. The results show that all the correlation coefficients, R, are larger than 0.95, and B§0, which is consistent with the results deducted from the above equation. It is recognized that the Tianheyong basalts resemble the product of Non-Modal partial melting, and the ratio between REEs (K-values) imply that the source rocks are probably different from primitive mantle. Fractional crystallization and partial melting processes can be distinguished on the basis of minor difference in behaviors between strong and medium incompatible elements. In the figure of (Ce/Yb)N vs CeN[38] (Fig. 9), fractional crystallization of plagioclase, olivine, and pyroxene under low pressure can not result in the significant changes of the value of (Ce/Yb)N, but the value of CeN increase rapidly in the evolved magma. If the garnet presents as a high pressure fractionated phase, it would result in the depletion of HREE and K2O. Figure 9 suggests that the Tianheyong basalts are plotted on the top of the partial melting curve indicating the extremely low degree of partial melting, and the majority of Cenozoic basalts from the adjacent regions are the products of less than 5% partial melting. Some trace elements show compatibility to part of minerals in peridotites; however, its bulk partition coefficient in the
Fig. 9 (Ce/Yb)N-CeN diagram for Tianheyong basalts[38]
Fig. 10 Th-Sc diagram for Tianheyong basalts ķ indicate fractional crystallization processes with different DSc; ĸ Indicate
partial melting processes with decreasing DSc
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
during partial melting. The value of La/Yb decrease more rapidly with the increase of partial melting degree, which suggests that the PYb values are increasing, which in turn implies that there is increase of the relative contents of garnet in the melting phase assemblage. Combined with the variation of Sc, we can speculate that the relative contents of garnet and clinopyroxene increased and decreased, respectively, during melting processes, which indicate that the Tianheyong basalts must be the products of Non-Modal partial melting more clearly. 5.2 Components of source region and magma generation
The significant variation of partition coefficients of REE primarily depends upon the contents of garnet and spinel rather than the variation of the contents of olivine, clinopyroxene, orthopyroxene, and pressure, temperature etc. during the partial melting of mantle peridotites (Table 3). Different melting degrees of spinel and garnet-bearing sources can be distinguished by their different partition coefficients for
La, Dy, and Yb (Fig. 11). When the partial melting degree is lower at the presence of garnet- or spinel-facies in mantle, it would cause the value of Dy/Yb to change in a very small range. And La/Yb value would change significantly in the melt when garnet-facies alone is present in the mantle, but the spinel-facies alone in the mantle does not. When both garnet and spinel involved during partial melting of the mantle peridotites; the value of Dy/Yb and La/Yb would have a wide range of variation (Fig. 11). Figure 11 suggests that the Tianheyong and the adjacent region basalts can not be generated in the primitive mantle (Fig. 11a), which coincides with the linearity simulation according to the deduction from equation in 5.1.2. But in the enriched mantle with̚1% partial melts from garnet lherzolite and in 2%–5% partial melts from spinel lherzolite, when mixed in different proportion can produce the basaltic magma of Tianheyong and its adjacent region. Most of the melts are of melting degree less than 5%, which is consistent with the result of Fig. 9. Both garnet and spinel perodotite source rocks are involved,
Table 3 Some trace elements partition coefficients for several major minerals in the basaltic melts Mineral phases
DB Sc
Th
La
Dy
Yb
Ol (Olivine)
0.12–0.3[39]
2.4×10í6–1×10-5[39]
0.0004[40]
0.007[36]
0.021[36]
Cpx (Clinopyroxene)
1.31[41]
0.012[42]
0.0536[42]
0.442[42]
0.43[42]
Opx (Orthopyroxene) Gt (Garnet) Sp (Spinel) Pl (Plagioclase)
0.213–1.501
[43]
[45]
0.04–0.1
1.57×10 –2.93×10 0.00137
2.62
0.36–0.57
í5
[47]
[48]
[45]
í4
3×10 –6×10 [40]
0.01
[40]
– 0.13–0.19
-5[39]
0.01 [49]
0.022
[50]
í4[44]
0.022
[40]
0.08–0.114[44]
[40]
4.03[45]–6.6[46]
[40]
0.01[40]
1.06 0.01
0.055
[40]
0.004[50]
Fig. 11 Diagram of (Dy/Yb)N vs (La/Yb)N for the basalts in Tianheyong and its adjacent region[51] Chondrite normalized value from[21]. (a) Melt curves use a primitive value from[21] where: w(La)=0.687×10í6, w(Dy)=0.737×10í6, w(Yb)=0.493×10í6. Under this circumstance the melt can not intersect the spinel lherzolite melting curve, nor can the garnet lherzolite melting curve accommodate the Tianheyong sample. (b) Partial melting curves of an enriched mantle, assuming that La and Dy with 20% enrichment, Yb with 10% depletion, which are: w(La)=0.824×10í6, w(Dy)=0.884×10í6, w(Yb)=0.444×10í6. The dashed box in the lower part of the diagram represents the enlargement of spinel lherzolite melting curves, and indicate all the basaltic magma with ̚1% partial melt from garnet lherzolite and 2%–5% partial melt from spinel lherzolite, and are mixed in different proportion respectively. The melting curves are calculated by using the non-modal batch melting model of garnet- and spinel-facies lherzolite: Garnet lherzolite: 0.598 Ol, 0.211 Opx, 0.076 Cpx, 0.115 Grt, melts with mineral assemblage of 0.05 Ol, 0.2 Opx, 0.3 Cpx, 0.45 Grt; spinel lherzolite: 0.578 Ol, 0.27 Opx, 0.119 Cpx, 0.033 Sp, with mineral assemblage of 0.1 Ol, 0.27 Opx, 0.5 Cpx, 0.13 Sp[52], the partition coefficient of La, Dy, Yb for different minerals same as in table 3. Legend same as Fig. 3.
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
which suggests that the basaltic magma might be originated from various depths of melting. The phase diagram of Fig. 12 on the basis of contents of CIPW of the basalts indicates that the Tianheyong basalts are the deepest in origin with wide variation range. The average depth is 100 km, similar to that of the Cenozoic alkaline basalts. If the source depth of basalts could represent the lithospheric thickness, it can be recognized that the lithospheric thickness of Miocene in Tianheyong, Hannuoba, and Jining areas are different from each other. However, it is hard to ascribe the thickening or thinning of the lithosphere to the differences in spatial distributions of basalts or to the spatial heterogeneity of lithospheric thickness. But the source depth of the Neogene Datong basalts decreases markedly, or at least, it might indicate the thinning of lithosphere from Paleogene to Neogene. 5.3
Petrogenesis and deep processes
Based on the above mentioned, it can be considered that the Tianheyong basalts represent the deepest primitive basaltic magma produced by the upwelling asthenosphere as compared with the temporal and spatial distribution as well as the petrological, geochemical attributes of alkaline basalts from Tianheyong, and adjacent regions. The source regions of the Tianheyong basanites are different from those of depleted asthenosphere and primitive mantle, which show enriched mantle features, such as the peak values of the HFSEs as: Nb, Ta, and Th. The enrichment feature was prior to the originating of the magma in source region. The enrichment of trace elements might be controlled by accessory mineral and fluids. The high contents of Nb and Ta might be resulted from the Ti-enriched mineral of rutile or (and) hornblende because of their high compatibility to the two elements[54]. The variation in concentrations of Ba, Rb, and Sr and their intersection phenomenon might be related to fluid phase
Fig. 12 Diagram of (Ne+3/5Ab)-(Ol+3/4Hy)-(Q+2/5Ab+1/4Hy) for the basalts in Tianheyong and its adjacent region[53] Legend same as Fig. 3
mineral of phlogopite or hornblende. Rb/Sr(0.01–0.02)[22] value similar to the primitive mantle, Ba/Rb(12–35) value higher than primitive mantle (̚11)[22]and the fractionation might be obtained from the fluid phase mineral of hornblende[55], indicating that it had undergone fluid metasomatism. In addition, if the source material are the melts, which were previously produced by partial melting of mantle also can result in the enrichment of incompatible elements. But this melt should not be generated by primitive mantle only for the ratio of incompatible elements, which should be similar to the primitive mantle value. The enrichment of the source regions should have been metasomatized by the fluid before the Tianheyong basaltic melts were produced. The origin of the fluid might be derived from the asthenosphere or deeper sources. The variation of alkali metals suggest the influence of the fluids after the formation of the melt. As compared with the K2O contents, the Na2O contents vary more significantly, which imply that the composition of the fluid are more enriched in sodium. This is supported by the K-Na cohesive fluid inclusions found in the high pressure and high temperature experiment of basanites from the east of China[56]. It is suggested that the source of the fluid might be diverse. Both the asthenosphere and the detachment layer[57] during magma enrouted to the surface can be the site for originating fluids. The correlation of some chemical features of elements in the Tianheyong basalts with the spatial distribution imply that the melting degree of north part basalts are lower. The negative correlation of CIPW norms of Ol and Ne display the high pressure melting processes. The significant variation of Ba/Rb and Na2O in the north part basalts suggest the fluid metasomatism, which might be related to the relatively low contents of Cr and Ni. As discussed above, the particular variation of Sc contents and La/Yb value are the result of the Non-Modal partial melting. So, we can conclude that the lower degree melts in the north might have transported to the earth surface rapidly after its generation, and the first erupted ones melts with high degree of silica undersaturation and more enrichment in incompatible elements, which had resulted in the decrease of relative contents of fusible clinopyroxene in the source residual mineral assemblage, and are ready to be affected by the fluids significantly. Given the discussion above, the north-south trending Tianheyong basalts are probably the products of melting processes in different depths, and the melts generated in different pressure are not mixed homogeneously, which is consistent with the rapid eruption onto the earth surface. The chemical composition of the basalts from the south are not constituting a continuous variational trend with the northern ones, which is probably because of the compositional diversity in the source region. Although the volume of the north-south trending Tianheyong basalts are very small but might not be generated from “a single point”, more likely from a wide range melting
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
pressure. The generation processes of the Tianheyong basalts might correspond with one of the models illustrated in Fig. 13. The primitive magma might erupt from several volcanoes (Fig. 13a) or from one single volcano (Fig. 13b, c) with melting processes in different depths. If the magma originated from the top of asthenosphere, the north part magma should have the highest originating pressure according to the geological setting. However, it is not consistent with the actual magma originating pressure. The pressure in the north is less than 3 GPa and south more than 3 GPa. In addition, the Tianheyong basalts are distributed along the river valley, which suggests magma might flow from south to north after eruption. Given these geological facts we conclude that the Tianheyong basalts might represent the products of polybaric melting column processes (Fig. 13c). Another important geological implication of the primitive magma is contributed to understand the deep processes and geodynamics of the mantle evolution. The model of mantle plume has long been used to explain the petrogenesis of Large Igneous Provinces (LIP), since it was first proposed by Morgan[58], such as Emeishan of China[59], Deccan of India[60], Nakusiak province of northwest of Canada[61] and Ethiopia[62] etc. There are always some typical evidences occurring in the mantle plume-derived LIPs, namely, (1) high temperature picrites and komatiites; (2) Hotspot trail; (3) High 3He/4He
ratio; (4) Uplift before the eruption; and (5) Short-term large volume of mantle-derived magma[63]. Because of the restriction of investigation techniques and influences of the later geological processes, these geological evidences can not always be identified simultaneously, which probably are the reasons why some researchers doubt about the existence of mantle plume. Given the characteristics of the Cenozoic basalts in this region, it seems that there is no abundant evidence to confirm the existence of mantle plume. The primitive magma in this region are the products of low degree partial melting of enriched lherzolite under relative low temperature, in contrast to the high-temperature, high degree of partial melting mantle-plume derived picrites, and komatiites. Both alkaline basalts and tholeiite were mainly controlled by partial melting processes rather than fractional crystallization of picrites or komatiites. So the primitive magma should be generated by decompression melting of up-welling asthenosphere. However, another important question is: what is the motivation of the upwelling asthenosphere? The edge-driven convection proposed by King[64–67] in recent years might provide some important implications for the geodynamics of the basaltic magma origin. The mechanism is usually related to the heterogeneity of thickness and property of lithosphere, which might occur in the transitional region between thick continental-lithosphere
Fig. 13 Schematic diagram of the genesis mechanism of Tianheyong basaltic magma and deep geodynamic processes of the Cenozoic asthenosphere in this region (a), (b), (c) represent three different magma genesis mechanisms for the Tianheyong basaltic magma, where polybaric melting column processes in (c) better coincide with the actual characteristic of Tianheyong basalts. (d), (e) represent the petrogenesis and deep geodynamic processes of the Cenozoic asthenosphere in this region. DT–Datong; HNB–Hannuoba; THY–Tianheyong; JN–Jingning; FS–Fanshi; YX–Yingxian.
YANG Zongfeng et al. / Earth Science Frontiers, 2009, 16(2): 090–106
and thin oceanic-lithosphere, or the edge region of old thick lithosphere in craton[67]. The discovery of Siziwangqi shoshonites indicates the delamination thining of the lithosphere around Tianheyong areas in Mesozoic[68], and the source depth of the Cenozoic basalts suggests that the “hotspot” locates at the thin edge areas of North China Craton, the distance between Erdos craton, and “hotspot” is consistent with the value of 600–1000 km calculated by King[67]. From the temporal and spatial distribution of magmatism, the 40–50 Ma mafic dyke implies the convection to start to reinforce in early Cenozoic[69]. The eruption of the Tianheyong primitive basaltic magma and Hannuoba Jining large volume basalts indicate the strong upwelling of asthenosphere. Therefore, the most probably induced mechanism of the magma genesis should be the edge-driven convection (Fig. 13d, e). The Tianheyong primitive basaltic magma is small in volume but with important geological implications. Furthermore study of temporal and spatial variation of basalts, and of the peridotites and xenocrysts brought up by the basalts would contribute to understand the magmatism, mantle characteristics, and geodynamics more clearly.
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(1) The Tianheyong basalts are primitive basaltic magma generated by low degree partial melting of enriched lherzolite( ̚
