1
2
3
Supplement D: Argument for the simultaneous unblocking
of titanomagnetite and pigmentary hematite
We have isolated two components (ITC and HTC) with maximum unblocking
4
temperatures of 560-580°C and 670-680°C during thermal demagnetization for most
5
samples in group C-E1d-L and E-E2n-L (Figure 3). The definition of these magnetic
6
components is based on the unblocking temperature ranges. It is well known that
7
hematite has two principal modes of occurrence (specularite and pigment) with
8
different unblocking temperature spectra [Collinson, 1974]. The maximum
9
unblocking temperature of the detrital specularite can be as high as 680°C, whereas
10
the unblocking temperature of the pigmentary hematite can be as low as 350°C [Kent
11
and Opdyke, 1985; McCabe and Elmore, 1989]. We hypothesize that the secondary
12
hematite, deposited during the circulation of hydrothermal fluids and induced the
13
remagnetization of the volcanics, contains large amount of pigmentary hematite,
14
which means the unblocking temperature spectra of secondary hematite show overlap
15
with that of the magmatic titanomagnetite. This is supported by the thermal
16
demagnetization of the siltstone sample from group D-E2n-S, in which secondary
17
hematite as the sole magnetic carrier lost more than 40% of the NRM up to 585°C
18
(Figure S2c). It is thus very likely that a considerable portion of the secondary
19
hematite (with the same origin as that in the siltstone) in the volcanics will be
20
unblocked together with titanomagtite up to 580°C. An estimate of hematite
21
contribution to the ITC can be gathered by comparison of the percentage of NRM
22
decrease up to 585°C to titanomagnetite contribution to the SIRM. In Figure S2,
23
almost 80% of the NRM of volcanic sample NL18.6 from group C-E1d-L was lost at
24
585°C. This value is much larger that the titanomagnetite contribution to the SIRM of
25
~13% (Table S8). Volcanic samples NL36.1 and NL37.3 also lost ~60% of the NRM
26
at 585°C, whereas their titanomagnetite contributions to the SIRM are ~15% and
27
~27%, respectively. We thus argue that secondary hematite is not only the sole
28
magnetic carrier of HTC, but also the significant contributor to the ITC.
29
Furthermore, for volcanic samples from group E-E2n-L, large amount of
30
pigmentary hematite was also unblocked together with primary titanomagnetite
31
during AF demagnetization. In Figure S2e, almost 65% of the NRM of volcanic
32
sample NL37.3 was lost at 100 mT. This value is much larger that the titanomagnetite
33
contribution to the SIRM of ~28% (Table S8). AF demagnetization was usually
34
ineffective in isolating ChRM directions of volcanic samples from group C-E1d-L,
35
indicating that abundances of titanomagnetite and pigmentary hematite in these
36
samples are low. This is consistent with the SEM observations that secondary
37
hematite in volcanic samples from group E-E2n-L is usually much finer than that of the
38
volcanic samples from group C-E1d-L (Figure 7).
39
Therefore, we argue that it is impossible to discriminate TRM from CRM with
40
neither thermal demagnetization nor AF demagnetization because of the unblocking
41
temperature/field spectra overlap of magmatic titanomagnetite and secondary
42
hematite.
43
Simultaneous unblocking of titanomagnetite and pigmentary hematite could also
44
explain the similarity of ITC and HTC in direction. We infer that ITC is a mixture of
45
TRM and CRM, whereas HTC should represent CRM. The inclination difference
46
between TRM and CRM would be only ~20° (dip of the bedding)considering that no
47
significant tectonic shortening or N-S movement have ever happened on the Lhasa
48
terrane during the time interval of TRM and CRM acquisitions. Considering that
49
CRM contributes to both ITC and HTC and inclination difference of CRM and TRM
50
is not significant, it is therefore reasonable that the mean of the ITC directions and the
51
mean of the HTC directions are difficult to statistically distinguished; especially when
52
the noise induced by alteration of the magnetic minerals during laboratory heating is
53
also considered.
54
55
56
57
58
59
60
61
References
Collinson, D. (1974), The role of pigment and specularite in the remanent magnetism of red
sandstones, Geophysical Journal of the Royal Astronomical Society, 38(2), 253-264.
Kent, D. V., and N. D. Opdyke (1985), Multicomponent magnetizations from the Mississippian Mauch
Chunk Formation of the central Appalachians and their tectonic implications, Journal of
Geophysical Research: Solid Earth, 90(B7), 5371-5383, doi: 5310.1029/JB5090iB5307p05371.
McCabe, C., and R. D. Elmore (1989), The occurrence and origin of Late Paleozoic remagnetization in
62
63
64
65
the sedimentary rocks of North America, Reviews of Geophysics, 27(4), 471-494, doi:
410.1029/JB1094iB1008p10429.